Games that aren't release yet won't show up on https://itch.io/games, they'll only be discoverable through your profile, collections, and https://itch.io/devlogs.
From an SEO perspective, it shouldn't matter. For the game's ranking on itch.io, the time you make files first available will count as when your game is launched. Future "major update" devlogs can update your game's freshness and help you stay higher up on itch.io's internal ranking.
So, I recommend creating pages early so you can get followers for your account so when you launch your game you can have an initial audience.