Yeah, rather than spend a whoooole lotta developer time trying to build their own forum software into the iNat website, they moved from using Google Groups to this Discourse platform in early 2019. https://groups.google.com/d/msg/inaturalist/72l44kifzsM/7-OLse_YAAAJ
And the option to be a member of the forum but not of iNat is just a feature of that solution, I guess. Yes? Or was there an actual decision to leave the forum open to non-iNat members? It’s an interesting aspect of the culture.