Agreed. I believe that this is the most likely option. We already have lore for Giants in Volo's (yes, it's being discontinued digitally, but we don't know if they're doing that physically), and there just aren't enough Giants in the history of D&D to justify a book like Fizban's for Giants.
Fizban's, the book made primarily by James Wyatt, was also the book that introduced most of what we know about the First World. And, in case you haven't seen the interviews with him about the book, he seemed really excited about the concept and talked about it for quite some time.
There was no "player stuff" in Storm King's Thunder, unless you count the magic items. No races, no backgrounds, no subclasses. What are you referring to?
It doesn't matter. It matters just as little as the fact that Eberron exists in the Great Wheel matters. It's completely useless to the people that don't use it and can be ignored extremely easily, and it's a great concept giving quite a lot of people exactly what they wanted for the rest.
Whether or not the First World actually existed is up for debate both in setting and in the real world, and even if it's confirmed that it did, it doesn't matter to the rest of the D&D Multiverse except to explain a few meta-questions (why so many of the same creatures appear in very different settings).
Did you miss 4e, or are you purposefully ignoring Nentir Vale/Nerath? The World Axis Cosmology is still very popular, even though it's not the base cosmology of 5e. Some of its planes (Feywild and Shadowfell) were even integrated into the Great Wheel due to how popular they were.
Or Fizban's, where they completely reinvented the concept of Great Wyrm dragons to make them more than just "Bigger Ancient Dragons", and introduced the idea of the First World, and introduced quite a few new monsters, too. Or the several new adventures that they've come up with since 5e's start (a lot of them are pretty bad, but there's still a lot of creative/inventive stuff in them).
Wtf does "creativity" mean to you? Does it exclusively mean "the ability to make new D&D settings" while excluding the fact that they were the ones that created Nentir Vale, the Feywild and Shadowfell, and have announced their plans to publish 2 completely new D&D settings in the next couple of years?
Why would you need to own Fizban's in order to use a First World setting book? I seriously doubt that there is literally anything in that book that would be necessary to play in a First World setting. Tasha's Cauldron of Everything talked about the First World before Fizban's did and the revisions to the Vistani before Van Richten's came out, and it's not like you need to buy TCoE to understand either of those books.
. . . Are you not familiar with Eberron? Or Ravnica? Or Theros? Or Spelljammer?
I see absolutely no reason why a First World or Prehistoric Setting book would be a "Standard Ren-Medieval D&D Setting with Dinosaurs and more Giants".
I mean, sure, there are a lot of "standard, pseudo-medieval D&D settings out there" (most created by TSR, not WotC, btw), but there are also quite a few major exceptions, and there's no reason to assume that WotC would make the First World/a Prehistoric Setting be one of those, based on the very concept alone.
How are Dinosaurs related to Giants? I'm pretty sure they've been stated to be related to Dragons in past editions. Sure, in 4e, Primordials/Elementals/Titans were related to Giants, but wouldn't that also point to this book being a "First World" setting book that takes place during the Dawn War?
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