Honestly, I'm not even sure. I've consumed a lot of content alleging that it is, but all they have to show for it is coronavirus pandemic numbers from when businesses were laying people off and firing them due to many being forced to close and all that stuff.
When the reopening happened, according to some of these creators, individuals who had been forced out of work simply refused or did not want to return.
If it is, I sure participated in it, but not by virtue of anything other than my own crypto success. lol
Now, all that being said, my personal opinion on the "Great resignation" theory is that it's just a theory that capitalists have that, to them, represents market FUD.