Heh, my first thought was “Don't they do this already?”, but apparently GrapheneOS was ahead of the curve there. Nice.
Not really. Samsung was the first with this, but their reasoning had absolutely nothing to do with security. It was because their phones slowed down over time and their solution was to give users the option to reboot it at specific intervals. You could even make the argument that the Samsung solution is still the superior solution because you get to set the interval.
I was admittedly confused about this distinction at one point too. It's a trade-off (although few people effected by this own phones with truly free, user-respecting soft/hardware in the first place).