Except that is the answer. You don't know how copyright works. This is understandable if you aren't from the US, I suppose, but it still doesn't excuse your refusing to listen to what people are explaining to you.
DMs Guild has no impact on the ownership of those settings, nor does WotC selling earlier edition PDFs have any impact on whether they open up those settings for DMsGuild creators. Because they own these things outright, they set the terms. You can't create Dark Sun adventures for DMsGuild not because its ownership is in question, but because they aren't letting you (probably because they don't know precisely what they want to do with it).
If you don't believe me, open up any Dark Sun (or other setting) book to the copyright page and read it. That should be enough explanation right there.