With a reverse vest, you own the shares immediately, and the vesting schedule defines how much of the shares you retain if you leave (i.e. if he quits now, 100% will be clawed back).

Yep, which means he effectively doesn’t own them right now, in any meaningful financial sense. For the purposes of control, sure.

The parent comment of this thread was talking about keeping 40% of nothing.

There's a decent chance that the other founder is in the same position, in which case the investor is the only one with a real stake in things