Except they didn’t. In fact the Roman Catholic Church as the full church in ecumenical counsel never made a pronouncement on the canon until the council of Trent in the 16th century. In fact the first list of the canon as we now know it was first written down by Athanasius (a Patriarch of the eastern church) in his 39th Festal letter in 367 AD. This was before the counsel of Carthage, a regional counsel of the local Roman Catholic Churches which accepted Athanasius’ list in 397.
Actually, the council of Florence, a bit more than 100 years before Trent (and ~50 years before Luther), listed the same canon as Trent did.