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.
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.
