Well, fir sure, but theybseem to have leaned into sharing stuff particularly hard this year.Having a default setting they can get the public to firmly identify as being "D&D" when that name is used helps when it comes to making future movies, cartoons and videogames and is them leveraging something they own that is not also shared via Creative Commons.
I think WotC is almost certainly going to go whole hog on the Great Wheel setting from here on out.
IANAL, but because of DTRPG/DMsGuild, the older Manual of the Planes is still "in print" for legal trademark purposes. So, no, a third party from outside of the DMsGuild could not use the name. The internal to DMsGuild standard is that directly using 5E book names is a no-go, but older Edition stuff is open as long as it is generic or under one of the opened Settings.It's on DMs Guild. It's not outside of WotC.
I'd love if one of the 10 million lawyers ENWorld turned out to have posting here last winter could chime in on whether "Manual of the Planes" is generic enough for anyone to use without WotC's permission, though.