He directly says "They are racing straight to self-improving superintelligence and gambling with our lives", that's a claim of imminent threat to humanity. If he truly believes that, direct action shouldn't be off the table nor even be immoral. Obviously he might not have the temperament or simply be a pacifist, that's totally normal but the point is that tweets are weird even then.
Why just make some tepid tweets if you think humanity is at risk? Why doesn't he leak documents and messages with the unfiltered opinions of leadership? Why isn't screaming "we are going to die" in CNBC? You'know, try anything?
That's where the "take it with a grain of salt" is important, maybe this is just a jumping point to a better job in the next super safe AI lab (for realsies this time)