There's a third theory of why they're doing this that I don't see discussed too much.

Behavioral tagging of bot traffic is actually easier when you're not serving plain HTML pages, because the number of signals thrown off by bad bots that don't exactly replicate browsing habits goes through the roof. The larger anti-bot implementations do a lot of this type of thing nowadays.

Bot authors no doubt have their own incentives for using old.reddit (lower bandwidth, conceptually simpler to crawl, etc) but it would not surprise me to learn that Reddit wants exactly one door for bots to have to shuffle through. They also don't work on old.reddit much at all these days, so any effort they have to put in to handling bot traffic is kept to one route.

Pretty dumb era of the internet we've wound up in.