No one mentions it because this 'fact' is both self-evident and is in no way specific to Signal or any other piece of software. If I can own the platform then

No one mentions it because this 'fact' is both self-evident and is in no way specific to Signal or any other piece of software. If I can own the platform then there is NOTHING you can do to stop me from owning all of the information you put into that platform. It has nothing to do with any specific tool you use on the platform and any attempt to tie this fact to a specific tool or program is an indicator of an attempt to muddy the waters (most likely so you can try to sell some 'solution' that does not actually solve the problem.) In almost all cases these so-called solutions end up being so inconvenient and painful to use that people actively route around them using mechanisms that are even less secure than the original tool that is being 'fixed' -- there is a reason no one uses Qubes and why everyone tries to avoid a SCIF if possible...