Do you compile your own open source client for your phone? Or do you install it from an app store? Most people are going to install it from the app store, so I believe my point still stands. What correlation is there between what's in the app store vs what is published in the open source repo? e.g. how do you validate that the app store client was compiled from a specific commit in the open source repo?
The Signal Android app has (had?) reproducible builds. You can see what is on the site and in the App Store is the same as when built from source. One person doing this provides some confidence for everyone else.
Still, I’m less confident in saying any other company fulfill’s Matter’s promise than saying they aren’t.