I'm going to go out on a good faith limb here, and postulate that OP may have added Gab, Odyssee, et al because they're not yet completely walled gardens, and it's easy to integrate with their APIs for discovery without hitting API limitations.
If I was building something like this, I'd start with something I use a lot (like Youtube), even if it required screen scraping, and then I'd move on to everything that had a dead simple API without low limits.
BUTTTT looking at the code, it would appear that they're screen scraping everything. But maybe it just so happened that the shape of Twitter's DOM is really really close to Gab's and it was super easy to add.
Or maybe this is a totally trojan horse approach to getting HN users on Gab. Who knows? But at least there's an easy way to turn it off. Not going to stop me from trying the extension, at least. (And as someone who tries to expose themselves to content bubbles that I consider insanely off-base, whether for education or entertainment, maybe I'll leave it on to see what kind of insanity is going on over there.)