They have confirmed that the book will be available separately, though it's premiering in the gift pack.
The weird part there is that the carnival is a minor note in the book: the core of the book is 5 pretty seperate 32-page modules, and only the intro module is carnival related. The other 4 are very, very different.
I fully expect to see all 3 in the next few years.
Re: available separately, it's totally not okay with me for them to put in the gift pack FIRST. It should be available separately FIRST, then in a gift pack, otherwise it's just a direct and crappy attempt to rip off desperate nerds, which like, no, do not do that.
I think in this case it seems like maybe it was a screw-up, from their apologetic attitude, like, maybe due to the pandemic completely screwing with publishing, they somehow managed to produce the ones for the gift pack but not the rest, or only were able to ship the gift pack ones or whatever, so I'm willing to cut them a break. Especially if it goes on D&D Beyond and so on at the same time as the gift pack - in fact that'll kind of prove is this is an honest or dishonest move - if it doesn't go on on Beyond when the gift pack comes out, that's pure dishonesty and attempt to manipulate customers in a very base way.
If they make a habit, in future, of bringing out books in more expensive, or gift-pack-only formats, months before they're available in normal editions (and/or digitally), then okay WotC, you can take a nice long walk off a very short pier thanks, one more time and that's literally the last WotC book I'd buy under current management. It's not something positive on any level.
Interesting re: carnival, they sure pushed that angle pretty hard - I guess a lot more people like that aesthetic than dislike it (The Greatest Showman did ludicrously well despite being a terrible film full of dodgy songs and with awful messages, albeit some of that was small children love it but still) - that actually increases my interest.
If they do all three settings they will get quite a lot of money out of me. I might even go back and buy Spelljammer if they do both PS and DS.