The other difference is that Eusebius had the tacit consent of the Pope.
And so did Lefebvre, despite the "no" of the Pope.
Why?
Because in necessity, that consent is
owed:
"Regarding seeking permissions from the superior, Suarez explains (speaking precisely of the pope) that here, “it is not a question of interpreting the will of the superior, but [a question] of his power” in order to know what is not necessary to ask the superior, because it is permitted to make use of “doctrinal rules” or “principles of theology and law,”
22 given that “one knows with more certitude the power [of the superior] which is not free, rather than his will, which is free [emphasis added].”
23 For that reason the subject, having prudently examined the circuмstances and been informed by the “doctrinal rules” or by the “principles of theology and law” that is “beyond the power of legislator”
24 to bind anyone to respect the law when it causes grave harm to so many souls, and that to obey in such a case would be “evil and a sin,”
25 he may not - indeed, he must not - submit to the law or to the command“on his own authority,”
26 “by his own judgment.”
27 Hence, by his own initiative, he refuses submission “without recourse to the superior,”
28 that is to say, without any dispensation or approval on the part of the said superior. The reason, writes Suarez, is:
that in such a case the authority of the superior cannot have any effect; indeed, even if he were to will that the subject, after having had recourse to him, should observe the law, the latter would not be able to obey him because he must obey God rather than man and hence in such a case its is out of place (“impertinens”) to ask for permission.
29
https://www.sspxasia.com/Docuмents/SiSiNoNo/1999_September/The_1988_Consecrations.htm