By that standard, it can never be verified because what is running and what is reviewed could be different. Reviewing relevant elements is as meaningful as reviewing all the source code.
Or they could even take out the backdoor code and then put it back in after review.
Let’s be real: the standard is “Do we trust Meta?”
I don’t, and don’t see how it could possibly be construed to be logical to trust them.
I definitely trust a non-profit open source alternative a whole lot more. Perception can be different than reality but that’s what we’ve got to work with.