The Danmaku!! thing is hilarious because it seems to be one of the things I said that they actually ended up sort of understanding, but they present it in the video as some sort of special snowflake case and then make up a scenario that obviously they think is ridiculous, when really it isn't ridiculous at all.
To get this straight, "you could make a chess game with Touhou characters as pieces and make it playable through a physics sandbox which is only available on Steam". Yes, as far as I can tell, this is entirely acceptable. It makes little sense for anyone to make such a thing the primary option for play since Tabletop Simulator in particular is expensive, and since nobody except TTS devs are getting money there isn't much incentive for anyone involved with the game to choose Steam anyways, but yes, that's a perfectly fine option. Danmaku!! only does it because it was low-effort to set up from a technical perspective and it lets people play across the internet, not because it was a truly good option. Simply put, such a game is not part of TTS, nor is it advertised in or alongside TTS, or anything of the sort. It's a game that's only available by being interested in the source in some respect... unless you distribute the resources for that custom game in wide-reaching channels itself (i.e. the resources that make up the game are still subject to the same rules as if it were a complete standalone product). There's no stopping anyone from making generic content like images and 3D models and then telling people you can import them into X program and do fancy things with it. This is also assuming you aren't charging to download those resources.
For another example, say Touhou Danmakufu wasn't related to Touhou whatsoever and wasn't free. You could make scripts for it, draw Touhou characters for sprites, and make a little game using Danmakufu. In the same way as above, it's a bit silly since you'd be offering your script for free but the platform costs money, which is prohibitive for any audience you're trying to get. But it's still an appropriate method to create and distribute.
If anyone from FSS is reading, here's your answer, I guess?