Genuinely trying to know if this is intentional to incorporate non-free open source software? If yes then why?
I don’t have a definitive answer to your first question, but why would we want to limit a sub to FOSS-only discussion? It’s a more restrictive designation. By calling the sub “open source” we’re keeping it open to software that isn’t technically FOSS.
I think a lot of people here actually mean foss when they say open source. Calling it foss might impart the right message and the importance of libre software to new folks. I am little conflicted about non-libre open source software as might be evident by now, maybe my bias is unjustified.
Wait until you see what the GNU/Linux sublemmy (lem?) is called.
Incorporate? Meh. People just can discuss all types of open source software here, not only free of charge, fully libre, etc…