And just to be clear, the reason it’s unclear is solely that such historical evidences for the Council of Nicea as St. Jerome’s assertion are not infallible, even if their historical value is unquestionable.
One last point:
The whole reason Luther argued against the canonicity of the deuterocanonicals is because they flatly contradicted his doctrine. And again, the Protestant issue with their canonicity is over whether there exists some distinction between an “ecclesiastical canon” and a “scriptural canon.”
But what of it?
A single passage of Maccabees teaches participatory atonement, prevening grace, prayer for the deceased and the existence of a sentient state in the afterlife that may not be defined precisely enough within Maccabees to be called “purgatory,” but certainly is neither Heaven or eternal damnation.
If one believes Protestant theology, Maccabees becomes perhaps the most damnable book ever written, the most grave moral threat to salvation comprehensible, the very defect of the Catholic church which Protestants believe Catholics can only be saved in spite of. So whereas a Catholic (such as St. Jerome) may find such doctrines hinted at in other books (1 Peter, Hebrew, James, Revelation, etc.), and could reasonably distinguish between a “scriptural book” useful for establishing the authority of the Church and an “ecclesiastical book” using for strengthening and teaching catechumens who have already accepted that authority, how can a Protestant begin to accept such a notion? Jerome’s alleged belief may be plausible to the readers of the Glossa Ordinaria, but how can it be plausible to any reasonable Protestant?