[grapheneos]

It's not an open source phone. It's closed source hardware with closed source firmware. The closed source userspace drivers/services could be rewritten but the hardware and firmware isn't in the same situation.

Hardware and firmware level protections can't be added by software.

Software features based on hardware security features also can't simply be added without those. Translating code to add a software equivalent to MTE could theoretically be done but would make the code require several times more CPU cycles and several times more memory. You would need far higher performance hardware and a bigger battery for that to be practical. MTE is what GrapheneOS uses rather than that.