Yes, it seems evident that it’s an argument of convenience, and not principle. They might as well just say that they hold to the canon they do on church authority, and leave it at that, but that is not a convincing argument to use if you want to level a charge against Protestants.
Protestants canonize scripture that is beyond dispute as scripture. Catholics take a broader approach and include some scripture that has been disputed historically, by Jews in some instances, and by some early Christians of note in others.
I can accept that without rancor. The disputed books were after all deemed good for edification even if not scripture by Protestants. Still are. What I have difficulty allowing to pass without comment is the frequent partisan claim that Protestants somehow just up and decided to cut out certain books of scripture, when in fact these had always been disputed as such in some quarters.