Something so clear it is one of the few papal uses of his "office" to dogmatically declare something had to have an opinion poll????
What a way to develop theology.
Well, since the premise for this dogma is that it was always believed from the beginning, then it is fitting that this premise be substantiated, which it cannot be (she just "forgot" it for a few centuries). However, the pope can autocratically declare that it was anyway. And in RC theology the protection from error that makes a declaration "infallible" only pertains to the declaration and not to arguments for it.
Which is how some Catholics basically argue, and it should be be understood that while Catholics will attempt to use Scripture to support distinctive Catholic teachings that are not manifest therein, yet for a faithful RC, the real basis for the veracity of RC doctrine is the premise of ensured magisterial infallibility. For Rome has presumed to infallibly declare she is and will be perpetually infallible whenever she speaks in accordance with her defined (scope and subject-based) formula, which renders her declaration that she is infallible, to be infallible, as well as all else she accordingly declares.