I'm always taken aback by this fascination and heroification with John Carmack (or similar figures) on HN. I love Doom and Quake too, but that doesn't qualify him any more than any other random senior engineer to work on something totally different. From his interviews it looks like he understands little about the technical details of ML, or about as much as anyone can learn in a few months, and is just banking on PR…
Carmack has proven his extraordinary technical skills. I recommend following his Twitter. Sometimes he posts non-obvious technical stuff. I read some interviews and to me, he doesn't seem to be a person who is driven by gaining popularity.
I think this news is very optimistic, as yet another intelligent, talented and hard-working person is joining the field. Moreover, he is a household name, which may lead to benefits like popularization of the topic, gaining investors attention and so on.
I will keep my fingers crossed for him :).