I trust smartphones with open schematics. Not because it's impossible to hide a backdoor but because it's harder.
This is just a strawman: Nobody claimed they were open hardware.
> Open schematics for a PCB don't make it any harder to hide a backdoor.
This is like saying that FLOSS doesn't make it harder to hide a backdoor. Of course it does.
So you are just attacking another FLOSS community with false [0] claims. This is suspicious.
[0] You can't say "extraordinary insecure" without specifying a threat model. For some threat models, GrapheneOS is less secure, e.g., https://news.ycombinator.com/item?id=45556788
Also, if I explicitly don't trust Google with anything, GOS is extraordinarily insecure for me until a new vendor appears.