I'm an employee, and I doubt he's hiding anything. Even while working as CTO, he has kept an ear to the ground in engineering and periodically participated in design discussions and other things. I really think he just genuinely enjoys engineering work. Internally I think many of us have great confidence in our current leadership, so it makes sense that Mitchell would feel like he can hand the reigns over.

I mean, many great open source projects are still maintained by the original authors. Why would it be suspicious for Mitchell to move into a better position for doing the same kind of work?

Yes, in most cases I’d suspect some foul play. In this particular case though the cynicism is probably unwarranted.