The more we talk about primordials and dinosaurs linked to giants, the more it makes sense that this is a First World book to me. If the D&D team really just wanted to make "Fizban's, but Giants," I don't see why there is any need to make them so explicitly tied to primordials or dinosaurs. It's just not necessary to make a giant-centric book. This UA however seems to do exactly that, so it seems at minimum they're going to expand the lore around the First World.
I'm just saying that why wouldn't they go full-hog and make a full prehistoric setting if they're already going to flesh out the concept more? Fits a niche that 5E hasn't filled yet, the subclasses fit the concept well. Matches how they're making new settings. I can definitely see the team mapping their cosmology for making the Multiverse, James Wyatt pitching this First World idea, and the team saying "Sure good idea, lets add that to our list of potential new books" and then Wyatt seeding the hint of it in Fizban's.