How can they be? They are subject to error. They are subject therefore to dispute. They cannot therefore be irreformable by definition, and therefore they cannot bind.
But St. Pius X disagreed - I don't see how you can get around this:
Wherefore we find it necessary to declare and to expressly prescribe, and by this our act we do declare and decree that all are bound in conscience to submit to the decisions of the Biblical Commission relating to doctrine, which have been given in the past and which shall be given in the future, in the same way as to the decrees of the Roman congregations approved by the Pontiff; nor can all those escape the note of disobedience or temerity, and consequently of grave sin, who in speech or writing contradict such decisions, and this besides the scandal they give and the other reasons for which they may be responsible before God for other temerities and errors which generally go with such contradictions. (Motu proprio "Praestantia scripturae")
The fallible decisions of the Biblical Commission (with an authority far less than that of the Ecumenical Council) are binding: "all are bound in conscience".