I think he's likely to trigger a lot of ire with this tweet. However, given what I know about his history, I think I know where this is coming from. Carmack himself did not finish school and was motivated early on to get involved in the profession of writing software despite lacking official credentials which entitled him to do so. I have a similar story and can relate. Therefore, I think what he's getting at is that it's important to just get started on something without waiting until you think you're "allowed" to. This quality of being a "doer" can stand out in colleagues. But of course there are many sides to the story. I don't think he meant to discount the others.

Update: I want to add that I think I recall being aware of figures like Carmack in my teenage years. Knowing of people like him, who were making lives for themselves by actively pursuing their interests, is I think a huge part of why I got involved in software early on in high school. It was a way for weird kids like me to have a sense of self-worth and accomplishment even if we were getting bad grades in more traditional subjects like "World History" or "Social Studies" on account of just not fitting in. And it was also at a time (in the 90s) when the economy highly valued such people going into the dot com bubble. So this tweet doesn't surprise me at all. I think the historical context I've given here probably describes Carmack's life as well and is really relevant in understanding his statement.