My guess is just that Reddit happily lets search engines crawl it, so that content is well-indexed, and because Reddit threads are often linked to from elsewhere the site is considered good quality.
I’d imagine Lemmy would eventually get to the same point naturally if enough information is shared here. At least, assuming it doesn’t block search engines.
Hmm although I don’t really understand how federation will fit with that, given it basically means the same content is duplicated on a bunch of domains.
Ooh wow. Mod sold out their subreddit.
Edit: found the thread. Bit of disagreement about what happened. Url for that link is here