"Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful."
The word 'definitively' makes this a definition.
The Power of the Keys was not given to the Church in order for it to subvert the truth, to declare what is wrong to be right.