It's hardly conclusive proof, but when we're discussing the ordinary magisterium, we aren't looking for a smoking gun, we're just looking for a consensus. It isn't so much about the passage itself (in Maccabees) as it is about what the Church's approved teachers have taken it to mean. Scripture is only a remote rule of faith, for understanding of it we look to the Church and her lawfully deputed authors. So, it's simply added to the rest of the sources affirming the existence of BoD.
I am demonstrating that the denial of the Holy Innocents and St. Dismas as a proof of BoD is untenable. The denial of the proof goes like this:
-The Holy Innocents and St. Dismas belonged to the Old Covenant
-Therefore, baptism was not necessary for them
-Therefore, they could not have received Baptism of Desire, which is a substitute operation whereby the soul is purged of original sin in place of actual water baptism
-Therefore, as non-recipients of BoD, they do not support BoD
I can use the same assumption (that one must be a direct example of such and such a thing to be a proof of such and such a thing) to show that Maccabees doesn't support purgatory:
-The Maccabees belonged to the Old Covenant
-The fathers in the Old Covenant who were justified only went to the Limbus Patrum
-Therefore, they would not have gone to purgatory
-Therefore, the prayers they offered for supplication of their dead do not support purgatory
But Maccabees is commonly regarded as supporting purgatory by commentators, showing that the commentators do not require one to directly experience what one's experience suggests, foretells, or implies.