CathInfo.com

Major:  +Tissier used to be a hard-liner.
Minor:  +Fellay used to speak like +Tissier.
Conclusion:  +Fellay used to be a hard-liner.

Minor is gratuitously asserted but not proven.  I don't understand why you don't see the logical flaw with this reasoning.  Do I have a hard-time believing that +Fellay used to be a hard-liner but changed?  Of course not, but I would like to see this established with real evidence.

But the other alternative is that +Fellay was soft (relative to +Tissier) even in the beginning, and perhaps even that he was an infiltrator put into the SSPX to bring it down.  

My point is, even if that whole argument is blown to bits, this argument is equally powerful in drawing a REAL conclusion you can act on:
+Fellay is soft today
There is no evidence +Fellay was ever anything other than soft.
+Fellay is a huge problem for Tradition
I don't believe this is necessarily the case -- but one of the two arguments has to be valid. And in either case, "Houston we have a problem!"

Whether in 2001 Bishop Fellay was a lion as Bishop Tissier de Mallerais was, I cannot say.

But that he spoke/wrote like one (as the author(s) of the Letter of 37 French Priests assert) cannot be disputed.

Ladislaus is correct to note that the author(s) ought to have supplied some evidence to demonstrate that contention (i.e., My guess is that they did not expect to be challenged on it, the earlier comments of Bishop Fellay being fairly well known within SSPX circles.  Also, because the Letter was being directed to Bishop Fellay, there was no need to cite his own words back to him to demonstrate what Bishop Fellay already knew:) That he no longer spoke as he used to, and, at least at that time, as Bishop Tissier still was).

Nevertheless, the following April 5, 2002 Letter of Cardinal Hoyos to Bishop Fellay provides several examples (provided by the Cardinal himself) of Bishop Fellay speaking with a tone and conviction similar to that common to old Bishop Tissier (and many of which are addressed not merely to SSPXers in conferences and interviews, but directly to the Cardinal himself).  It seems once upon a time, Bishop Fellay was not afraid to preach the truth to Rome (again, whether or not he believed his own words is another subject altogether):

https://www.catholicculture.org/culture/library/view.cfm?recnum=4650

[All citations are provided by Cardinal Hoyos.  This article is extremely difficult to find on the internet]

-"It cannot be denied that the dysfunction of the Catholic hierarchy . . . omissions, silences, deceptions, tolerance of errors, and even of positively destructive acts, reaches even into the Curia, and unfortunately even to the Vicar of Christ. These are public facts that can be seen by ordinary men." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21, 2001)

-"The Conciliar Church is like a termite that bores away from the inside. For 30 years and more, the same principles have been applied with an imperturbable coherence, despite their catastrophic fruits . . . So, we prefer to keep our freedom to act for the whole Church rather than let ourselves be isolated in a zoo of Tradition. It is necessary to shake up the Catholic world, which slumbers in a post-Conciliar lethargy." (Interview with Msgr. Fellay in the journal "Pacte," Summer 2001)

-"It seems to me possible to affirm, from our point of view, that, following Popes Pius XII and Paul VI, the Church is presently in a literally apocalyptic situation." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21, 2001)

-"For it is in this regard that can be found the novelties of the new theology, that were condemned by the Church under Pius XII, and that were introduced into Vatican II . . . They would have us believe today that these novelties are but a development in conformity with the past. They were already condemned, at least in their principles." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21,2001)

-"We are only a sign of the terrible tragedy that runs through the Church, maybe the most terrible of all until now, where not only dogma but everything is attacked." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21, 2001)

-"A Magisterium that contradicts the teaching of the past (for example, today's ecuмenism versus Mortalium Animos), a Magisterium that contradicts itself (see the Joint Declaration on Justification and the preceding note from Cardinal Cassidy, where one finds a condemnation of and also praise for the term "sister Churches") — here lies a haunting problem. Thousands and millions of faithful Catholics who [leave] the Faith are damned because of the failures of Rome, here is our concern." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21, 2001)

-"This crisis in the Magisterium constitutes a problem that it is almost impossible to resolve practically. Moreover, the nightmare concerns also the Curia and the residential bishops." (Letter from Msgr. Bernard Fellay to Card. Castrillon, Menzingen, June 21, 2001)

-"Rome approached us, saying: Listen, you have a problem; it needs to be solved. You are outside; you must come back in, under certain conditions. Now it is our turn to respond: No, it is not like that. If we are in the situation in which we currently find ourselves (a situation of being marginalized and persecuted), we are not the cause. The cause is to be found in Rome; it was because there are grave deficiencies at Rome that Archbishop Lefebvre had to adopt certain positions in order to conserve certain goods of the Church that were being vandalized." (Interview with Msgr. Fellay in the journal "Pacte," Summer 2001)

-"We reject the dilemma they are trying to snare us in again. It is very clear: we are not outside, nor will we allow ourselves to be caged." (Interview with Msgr. Fellay in the journal "Pacte" Summer 2001)

-"We are presently at a standstill, an impasse. I think that this stoppage stems from the basis on which the dialogue was begun [i.e., practical accord, rather than doctrinal]." (Interview with Msgr. Fellay in the journal "Pacte," Summer 2001)