I feel that perhaps "cult" is too harsh a word to be used here. The OTT is certainly a dense community, and any who don't appear within it for a while are referred to as "being in the Basement", which as you say implies that it's not possible to ever leave permanently. However, I feel that the Basement, as a term, is a deliberate choice of phrase for someone to use who feels they wish to continue being a part of the community even in absentia
due to other commitments, not a blanket term that applies equally to those who aren't here by choice and those who aren't by circumstance.
As for the tensions etc between OTTers and the rest of the fora, I feel that such is (as part of human nature) to be expected anywhere that there is a gathering of like-minded individuals within a broader group. Even now, for example, "gamers" are frequently looked down upon by "non-gamers" as wasting their time / being unproductive/lazy / being less contributive to society in general. Same with furries, Bronies, dudebros, sports fans, opera-goers, wine lovers, extreme sports, fishing, hunting, and certain fields within the scientific community, as a non-exhaustive list of examples (and I have no doubt that folks from every one of those subsets exist within the OTT). That the OTT is as welcoming as it is may be due to "the Sadness", or it may be a natural result of the personalities that have gathered here anyway; I believe it's the latter, although the former would have provided much-needed context.
You've noted that we are... Somewhat overzealous in welcoming people into the community. I'm in two minds (heh) about that; while I agree at a surface level that everyone jumps on the "Oh hi, welcome Revealer the Recent! Titles shall be bestowed upon thee!" approach (I was going to say "bandwagon", but that feels like an unintended criticism), I feel that this approach is far better than either stony silence, or worse, a "Blitz the entire OTT so you understand our culture or GTFO" mentality. To join in our community, you don't need
to understand it all (ever), and as a newcomer to the discussion, you could go entirely without using any OTTish at all in your interactions for the entirety of your stay and nobody would bat an eyelid. A cult would absolutely insist on linguistic indoctrination, eventually ostracizing those who refuse to partake. A cult would also actively attempt to recruit new followers; the OTT generally sits here waiting for newcomers to join if they wish, and leave if they'd rather, although I'll note that due to its activity level, it is always on Page One of the fora and thus has constant visibility. (Said visibility is also a potential cause of hostility from non-OTTers. I understand and don't begrudge them that.)
I don't feel that there is an "enforced isolation" here. I'm here by choice, I can interact with other threads and other fora as I see fit (or not, if I prefer) and noone else in the OTT gives a single molecule of waste management either way. I could declare that Huzzah I'm off now, goodbye! and I'd get a few farewell messages, but the OTT would continue without me just as it did before I joined. I envisage that if the OTT continues as it is, the entirety of the original participants will eventually have been replaced by newcomers, the dialect will have evolved some more, and the inclusive (but not insular) culture will still stand firm. It's an oasis in the Internet desert, a refreshing pool in a normally scorched landscape of casual racism, multiphobias, and the GIFT
Lastly, my apologies for I-jacking your thoughtful and well-written post (although it [of course] wasn't written for my benefit, I appreciate you taking the time to write it, since it was excellent food for thought). None of the above should be taken as Absolute Truth; it is simply a differing opinion, and is absolutely capable of being utterly incorrect, misaimed, inaccurate and/or bag-of-hammers-level dumb.