Popularity isn't a technical achievement in itself. But what is a technical achievement is that he built a visually impressive 3d engine that worked well enough for over a million people to buy it at a time when almost no one else was doing this. Minecraft was released nearly 10 years earlier--before Unity and Unreal had completely taken over.
I'm also not saying that the witness engine is somehow better than anything else out there, but it's a significant enough technical accomplishment that along with Jon Blow's other work, he has the experience to back up his opinions.
I've also never seen the critique that Jon Blow is just an influencer with no relevant experience from anyone with equivalent relevant experience. I've seen other experienced game devs who disagree with him, but I've never seen them say that he doesn't have the experience to have them.