Nudity is not considered adult only. It might be on itch, since they do not have any 17 rating or other mature content labels.
They vaguely tell you this: "If you have a project page that has strong adult themes, then it must be marked."
Whatever that means. What is an adult theme? What is an adult theme but not a strong adult theme? Is the graphic depictions of those themes relevant? And so on. And what is considered adult or strong adult would also vary with your location, but the developers come from all around the world with different cultural standards. You can show nudity in day time tv in Europe.
In practical terms, sexual stuff has to be marked. Nudity might need to be marked because of their equally vague not safe for work wording. Gore and violence might have to be marked, but looking at all those horror games, it might not be enforced for all but the most extreme games.
A notable non sexual example would be Fear & Hunger. It was released on itch and steam. But it never got an official esrb rating. Yet it is restricted on Steam and on Itch, maybe by the creator's own rating, maybe by people categorizing it.
Pragmatically, if youtube restricts streaming of the game, it probably has to be marked. And if not, then probably not.