Let's move to Holmes's translation to see if we can find any better evidence to determine whether T used the true or falsified against M. Book Four begins:
Every opinion and the whole scheme of the impious and sacrilegious Marcion we now bring to the test of that very Gospel which, by his process of interpolation, he has made his own. To encourage a belief of this Gospel he has actually4 devised for it a sort of dower, in a work composed of contrary statements set in opposition, thence entitled Antitheses, and compiled with a view to such a severance of the law from the gospel as should divide the Deity into two, nay, diverse, gods----one for each Instrument, or Testament as it is more usual to call it; that by such means he might also patronize belief in "the Gospel according to the Antitheses." These, however, I would have attacked in special combat, hand to hand; that is to say, I would have encountered singly the several devices of the Pontic heretic, if it were not much more convenient to refute them in and with that very gospel to which they contribute their support. Although it is so easy to meet them at once with a peremptory demurrer,8 yet, in order that I may both make them admissible in argument, and account them valid expressions of opinion, and even contend that they make for our side, that so there may be all the redder shame for the blindness of their author, we have now drawn out some antitheses of our own in opposition to Marcion. [4.1.1, 2]
Omnem sententiam et omnem paraturam impii atque sacrilegi Marcionis ad ipsum iam evangelium eius provocamus quod interpolando suum fecit. Et ut fidem instrueret, dotem quandam commentatus est illi, opus ex contrarietatum oppositionibus Antitheses cognominatum et ad separationem legis et evangelii coactum, qua duos deos dividens, proinde diversos, alterum alterius instrumenti, vel, quod magis usui est dicere, testamenti, ut exinde evangelio quoque secundum Antitheses credendo patrocinaretur. [2] Sed et istas proprio congressu cominus, id est per singulas iniectiones Pontici, cecidissem, si non multo opportunius in ipso et cum ipso evangelio cui procurant retunderentur; quamquam tam facile est praescriptive occurrere, et quidem ut accepto eas faciam, ut rato habeam, ut nobiscum facere dicam, quo magis de caecitate auctoris sui erubescant, nostrae iam antitheses adversus Marcionem.
Call me stupid but the only texts of M that T ever claims to have in his possession are (a) the Antitheses and (b) some letter which demonstrates Marcion held the Catholic faith. The whole reason the Antitheses are brought up is because it is at least something which demonstrates the Marcionite faith. The gospel of M is by contrast "hidden" and by T's own admission, not well known.
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote