Sadly it boils down to "trust us" (or really, trust wcathcart), which is a position users have been betrayed in countless times over the past decades (and Facebook has a horrible reputation for user privacy). Compare that with Signal—or any other application with an open-source client—where we can inspect the source code and compile our own client.
If this is done client side, it doesn't boils down to that. You can easily decompile and see for yourself what it does. You will gain quite a bit of notoriety if you are the first one to catch them too.
As he said:
> if we ever did it would be quite obvious and detectable that we had done it.
> Assuming your device allows you to get the binary. Apple is already in a position to disallow this if they choose to in the future.
Theses kinds of thing never stopped anyone. Being the first to share a hash of a system file in a console is always an achievement that many hackers tend race to do when a new one is released.
For sure the harder it is, the less person will do it, thus the more theses things will be able to go under the radar, but for now it's not so much an issue.
