The latter is not universal. Several popes have explicitly stated that they are indeed fallible (which would render total infallibility self-contradictory). Also, infallibility is generally limited in certain ways that I won't bother researching in enough depth to put into a concise description. But the pope can't just make up some random shit and have it automatically considered to be the word of God.
Of course, pointing out that someone violates the basic tenets of Catholicism while naming those specific tenets is well within "the pope is right" territory.
So your argument is that since you are opposed to the app's very existence it's immoral to test it for security flaws.
I'd like to argue against that with the principle of defense in depth. I'm also not a friend of OS-level age verification and would like it to be dropped. But if it is implemented I want it to be implemented in a way that isn't wildly insecure. I can simultaneously argue against the principle as a whole and insist that any implementation of it be secure. If it does come I at least want the damage from a botched implementation to be mitigated.
To use your cage analogy, I can both complain about the principle of caging people and about the fact that the cage is badly made and poses an injury risk to the people inside it. Neither is acceptable.