If you did "web stuff" in the early 2000s (like 2005-2010). You'd probably know who he is. He did Ruby on Rails, a backend web framework.
But that was also very Start-up and America focussed. So if you did web dev in some other country and didn't have colleagues who were into that culture you still might've missed the name.
TBH the biggest difference is him being more vocal.
I'm pretty sure most of the people who did "web stuff" at the time and used twitter (key point maybe) know him simply because you'd often see his tweets. Regardless of coutry (I'm from Russia, for exampl)
DHH markets himself much better. His company (basecamp), in a sense, revolves around his public persona and he's unapologetic about this. It's the same with all of his projects (e.g. Omarchy recently).
But that was also very Start-up and America focussed. So if you did web dev in some other country and didn't have colleagues who were into that culture you still might've missed the name.