Let's leave aside the strange statement - Principalem adversus Judaismum Epistolam nos quoque confitemur quae Galatas docet. Romans was placed first in the orthodox canons (for political reasons) perhaps as early as the end of the second century, the Marcionite canon might well have been recognized as agreeing with the original order of epistles in Catholic documents like the Muratorian canon.
The reason I hesitate to understand the Marcionite canon began with the Epistle to the Galatians is because there is so much evidence for a Corinthians first canon in Against Marcion. I don't know how to explain it. Here's one example. From Book Four:
.To sum up: if it is agreed that that has the greater claim to truth which has the earlier priority, and that has the priority which has been so since the beginning, and that has been since the beginning which was from the apostles, there will be no less agreement that that was handed down by the apostles which is held sacred and inviolate in the churches the apostles founded. Let us consider what milk it was that Paul gave the Corinthians to drink,a by the line of what rule the Galatians were again made to walk straight,b what the Philippians, the Thessalonians, and the Ephesians are given to read, what words are spoken also by our near neighbours the Romans, to whom Peter and Paul left as legacy the gospel, sealed moreover with their own blood
I defy anyone looking at the context of this statement as to the order of the epistles to argue that someone thought that Corinthians should be first and Romans last. Someone had this canon, but whom? I think there is evidence in Against Marcion to suggest it might have been Marcion.
