Alice Nothing is bulletproof. Not GrapheneOS, or anything else. Anyone trying to sell you on a 100% guarantee is being dishonest. As mentioned in the GrapheneOS documentation:
[...] The next line of defense is preventing an attacker from exploiting a vulnerability, either by making it impossible, unreliable or at least meaningfully harder to develop.
What MTE brings is substantial security improvements. The project claims that it is the biggest security feature being shipped since the project started in 2014. It really is quite significant.
GrapheneOS already heavily focused on defending against exploitation of unknown vulnerabilities, which is explained here:
MTE will significantly strengthen GrapheneOS' defenses and will raise the bar for exploitation even further.