Just because something makes sense and is consistent with everything else you know doesn't mean it is true.

When people say "that totally makes sense" they usually also mean "I now believe it", which is a jump in logic.

If something is true, then it is consistent. However the inverse is a fallacy called "affirming the consequent". If you determine something is consistent, that doesn't mean it is true.

Just because it is consistent to believe GrapheneOS has chosen only Google phones for technical reasons, that doesn't prove it is true. You have to determine veracity via other means.... like knowing the history of the project, the history of attacks on GrapheneOS phones, the likelihood of google phones having exfiltration hardware (and the risk of a state agency using it and getting caught), etc.