Burnout, which doesn't need scare quotes, very much still applies for the humans involved in building AGI -- in fact, the burnout potential in this case is probably an order of magnitude higher than the already elevated chances when working through the exponential growth phase of a startup at such scale ("delivery apps" etc) since you'd have an additional scientific or societal motivation to ignore bodily limits.

That said, I don't doubt that this particular departure was more the result of company politics, whether a product of the earlier board upheaval, performance related or simply the decision to bring in a new CTO with a different skill set.