Source availability is what makes a chain of trust possible that simply isn't meaningfully possible with closed source software, even with dynamic analysis, decompilation, reverse engineering, runtime network analysis with TLS decryption, etc.
Both you and the preceding commenter are correct that just running binaries signed and distributed by Alphabet (Google) and/or Apple presents room for additional risks beyond those observable in the source code, but the solution to this problem isn't to say "and therefore source availability doesn't matter at all for anyone", it's to choose to build from source or to obtain and install APKs built and signed by the developers, such as via Accrescent or Obtanium (pulls directly from github, gitlab, etc releases).
There's a known-good path. Most people do not take the known-good path. Their choice to do so does not invalidate or eliminate the desirable properties of known-good path (verifiability, trustworthiness).
I genuinely do not understand the argument you and the other user are making. It reads to me like an argument that goes "Yes, there's a known, accurate, and publicly documented recipe to produce a cure for cancer, but it requires prerequisite knowledge to understand that most people lack, and it's burdensome to follow the recipe, so most people just buy their vials from the untrustworthy CancerCureCorporation, who has the ability to give customers a modified formula that keeps them sick rather than giving them the actual cure, and almost nobody makes the cure themselves without going through this untrustworthy but ultimately optional intermediary, so the public documentation of the cure doesn't matter at all, and there's no discernable difference between having the cure recipe and not having the cure recipe."