I can't believe I had to scroll this far to find it this explanation.

My first thought was "wait did discord saying something bad about FOSS lately?" No, nothing unusual. Must be missing something.

Turns out some people see Discord as a sort of mascot for closed source, and that is what the joke hinged on.

Edit:

Actually it's worse than that, isn't it? They're seeing the coexistence of closed source as being antithetical to open source.

Good luck with that I guess.