Holy shit. This whole thread completely turned my respect for Jamie around. I've been a fan of his writing and projects since the early 2000s but this is such a fucking odd hill to die on. It's his hill though, so all I can do is speak out in support of your attempt to be helpful.
Jamie, I get that you despise anyone telling you how to do what you do (your fierce independence is something I admired about you), but in this case I don't feel that was what was going on. It was a suggestion, not a callout or a demand, and this is a public space that you chose to join, a space that is full of disabled folks who depend on alt-text to be able to even read a post.
Accessibility matters, you're old enough to know that, and now that I know your position on it (especially as someone who is legally blind without glasses and has a hard time reading tiny text even with them), and I have seen you punching down extremely hard and without compassion on fellow disabled folks, I don't think I can consider you someone to look up to anymore. If you've read this far, I know you will block me after if you haven't already, so I'll just say one more thing. How you choose to be known and remembered is exactly that: your choice.