This feels like a massive amount of projection to me. Yes, I'm sure Genndy was upset when projects of his don't perform well, as any creative would be. Though from what I heard Unicorn started out strong ratings-wise at least, but maybe it didn't finish that way. Either way with how much of a dumpster fire WBD has been since...well, ever, that has to be a contributing factor too. And more broadly, of course he's angry about the state of action animation in the US, and he should be because I am too. It's been dire for a long time, with only a handful of notable success stories over the past decade or two. Hell, I originally got into anime over 20 years ago because it scratched an itch I didn't even know I had because there wasn't anything remotely comparable being produced in the US.
As far as tying all of that into Primal, that's a pretty big reach. I think it's far more likely that Genndy realized he still had some stories to tell with Spear, but obviously the ending of last season precluded that, so he came up with an idea for how to bring him back into the picture. I wasn't sure what to think when I read that he'd changed his mind from the anthology concept (which I thought was interesting but ultimately not very compelling), but after this first episode I'm digging it. Spear's character was always about the balance between humanity and a savage force of nature unleashed, and this new season is pushing that to the extreme.