yes and no:

the copyright owner can do whatever they want, but they can't really revoke a GPL license. that's not really a thing.

and the part about

If you obtained your copy under the old license you can use it under the old license when you obtain a new copy you have a new license agreement.

seems to me like you are implying that "use under the old license" means "run the program on my own machine", but that's not true, since GPL explicitly allows redistribution and modification.

under a GPL license, you effectively give up control over your software voluntarily:

The GNU General Public Licenses are a series of widely used free software licenses, or copyleft licenses, that guarantee end users the freedom to run, study, share, or modify the software.

(highlighted the relevant portion for your convenience)

this makes revoking the license effectively impossible.

you could continue development under a different license, but that gets legally tricky very quickly.

for example: all the code previously under GPL, stays under GPL. so if someone where to modify those parts of the code and redistribute it as a patch, you couldn't legally do anything about that.

which seems to be what the OOP claims the change to a CC-BY-NC-ND forbids, apparently misunderstanding, that this new license only applies to code added to the repo since the license change, not the code from before the license change.