The subject of the thread is, if it's ok to only have the page link to another store.
No, it's not ok. Reasons why you can't put the game on Itch actually do not matter. If you only offer the game on another platform and just use Itch for promotion.
Imagine developers posting a project page that is only available on the Playstation store. Or the Nintendo store. Would you think those pages should exist on Itch? It's bait and switch, that's what it is. Show a shiny game to people, but if they want it, they have to go elsewhere.
Even all those demo only projects are borderline. You can't claim, that a game "is on Itch", if all you ever give people, is the demo version.
What do you think would happen in the reverse? Having a game on a store system, but not actually have the game on that store, but only an advertisement page or a demo version. Have you ever seen such a thing? And I am not talking about premium versions. I am talking about a storefront where a catalog item was not the thing, but only advertisement of the thing and you would have to buy it elsewhere. Is there any store that allows this? I am curious.
If you want a promtion version here, maybe you can export a web version of the game or something like that.
And if you want to ask, if it is allowed, instead of if it is ok: it is not ouright forbidden for every scenario. But it violates quality guidlines and can result in non indexation. If you do not have any files on the project that probably is automatic. And if you have fake files to cheat on indexing, well, that's sounds very problematic.
Make a blog posting for your followers, if you want. But do not make a project page, that is just a blog posting in disguise.