As Cryptonix  alluded too above, the question lies in a Latin Rite Catholic being confirmed by an Eastern Rite priest. They have no question of validity with an Eastern Rite person confirmed by the Eastern Rite priest.
 An Eastern Rite Bishop's Confirmation they would accept as well.

If this is the case I dont see an issue with their position.  The way their position is written however is as follows: "31. Confirmations done by Eastern rite uniates and by eastern schismatics must be conferred again sub conditione, unless they have been performed by a bishop." There is no mention of this only applying to Roman Rite confirmands.  In fact is explicitly mentions applying the rule to eastern schismatics, which seems very hard to justify. 

I know a gentleman for instance who converted from an eastern schismatic sect.  He was baptized and confirmed by his schismatic priest.   He is now a traditional Roman Catholic.  On what grounds should his confirmation be subject to positive doubt?  

I would argue in favor of Mater Dei/CMRI rather than MHT.  MHT enforces their theological opinions very forcefully which can result in a certain cookie-cutter formation and division from the greater Church.  Examples aside from those mentioned above are the banning of gothic vestments, the rule against any member of RCI being a Scotist, conditionally confirming those confirmed by a priest in the Eastern Rites (which has always been considered valid,) etc.

Oh, CMRI impose their positions also.  There's no group that doesn't.  Good luck going to CMRI seminary and insisting on using the pre-1955 Holy Week Rites, or being opposed to NFP or being a "Feeneyite" ... we've seen what happens.  There's no group that doesn't have theological opinions that you're not expected to be in conformity with.  In terms of vestments, the SSPX had periods where they banned the use of birettas by seminarians (equating them somehow with sedevacantism).  They would have done well to ban the lingerie-like ankle-length surplices worn by the Urrutigoity crowd.  I could see a group having various "standards" about certain matters, as you could imagine the chaos of excessive pluralism, where half the seminarians would wear birettas and the other half wouldn't.  As for conditionally confirming those confirmed in the Eastern Rites, you'd do well to explain their rationale.  It's not unheard of for various Novus Ordo priests to "transfer" into the Eastern Rite.  Bottom line, though, is that every group has various positions that you're at least publicly expected to follow as a member of the group, including CMRI.

VIII. Sacraments to Be Conferred Again Absolutely or Sub Conditione

24. As a general rule, no sacrament should be repeated sub conditione except where there is a positive doubt concerning its validity.
...
31. Confirmations done by Eastern rite uniates and by eastern schismatics must be conferred again sub conditione, unless they have been performed by a bishop.

31 seems to contradict 24.  Based on the rationale that Eastern Rite confirmations would be valid if conferred by a bishop, it seems that they deny that Bishops of the Conciliar Church have the power to delegate the authority for priests to confirm.  I would dispute that, since that's always been enshrined in Eastern Rite Canon Law as general rule and does not require an individual bishop to explicitly delegate to individual priests.

If this is the case I dont see an issue with their position.  The way their position is written however is as follows: "31. Confirmations done by Eastern rite uniates and by eastern schismatics must be conferred again sub conditione, unless they have been performed by a bishop." There is no mention of this only applying to Roman Rite confirmands.  In fact is explicitly mentions applying the rule to eastern schismatics, which seems very hard to justify. 

I know a gentleman for instance who converted from an eastern schismatic sect.  He was baptized and confirmed by his schismatic priest.  He is now a traditional Roman Catholic.  On what grounds should his confirmation be subject to positive doubt? 

.

Well, it's interesting that if the sacrament is conferred by a bishop they will automatically treat it as valid, even a schismatic bishop. It's only the confirmations conferred by a priest that they would repeat.

I don't know their reasoning, but if I had to guess, I'd say it's not certain exactly how confirmation conferred by priests works, and it's not certain that it's always valid. The eastern rite priests had some sort of permission from the pope, I think, which has not existed since 1958. So probably they are just being cautious and giving confirmation again just in case it's invalid without a papal indult when it comes from a priest.