That isn't fair. People need a break. "AGI" / "superintelligence" is not a cause with so much potential we should just damage a bunch of people on the route to it.

Why would you think burnout doesn't apply? It should be a possibility in pretty much any pursuit, since it's primarily about investing too much energy into a direction that you can't psychologically bring yourself to invest any more into it.

Software is developed by humans, who can burn out for any reason.

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.