> Is it because HAProxy's internals weren't designed to do that and that it would too big of a rewrite?

Bingo. The internals will eventually be rewritten to support things like this (and I think that's what Willy was hinting at towards the end of the article) in time. But it's a big project, and it's a complicated piece of software, and there are a lot of conflicting demands on dev time.

That said: it's a welcoming community, and if you want to help... do it!