Beleriphon
Totally Awesome Pirate Brain
You aren't giving the IP to WotC. What you are doing is allowing anybody else on the DMsG to use your monster in their work. So say I come up with the Brightened-Mangler-and-Carpeater monster and post it to The DMs Guild. What Morrus can then do is use the BMaC (
) in an adventure he publishes on DMsG. I still retain the rights to that monster, but by publishing it on the DMsG I license it to other people to use.
WotC can't take my BMaC and publish a "The Big Book of BMaCs and their Habits", since I own the IP for BMaCs. But, WotC might offer to outright buy the BMaC from me, but that's a different kettle of fish than the Dugeon Master's Guild.

WotC can't take my BMaC and publish a "The Big Book of BMaCs and their Habits", since I own the IP for BMaCs. But, WotC might offer to outright buy the BMaC from me, but that's a different kettle of fish than the Dugeon Master's Guild.