First came HBO Go. This was the online streaming for those with HBO as part of a standard cable package. Apparently it was built on a .Net stack and had huge problems.
HBO Now is the service created for standalone sales of streaming. Apparently it had a completely different tech stack because of issues with HBO Go. Apparently it came from the same people who did the streaming services for MLB (Major League Baseball)> In my experience it was very good.
HBO Max is the expanded and updated content ecosystem that came with the merger. I don't know what the tech stack here is. I suspect it's just incremental changes to HBO Now but that's just a guess.
Perhaps the commenter was referring to HBO Go's problems as evidence that serving VODs isn't "solved". To clarify, I mean "solved for Netflix".