I marked the game as 18+, but not as a game "with explicit content."
There is no distinction on Itch for that. There is 13+ because Itch is 13+ and there is 18+. Look at the front page (without being logged in, since your account will have adult games set to on, since you published one). Look at the horror games there. And then decide, if your game really warrants a 18+ marking. If you mark it as nsfw, you can't post in release announcement. It does not matter why your game is marked so.
If you have no nudity, no ultra violence, no adult topics, you most likely do not need that nsfw marking. Adult as in adult bookstore, not adult as in paying taxes.
Skyrim is M and that is 17+, without nudity. And that is an official rating. Blood and Gore, Intense Violence, Sexual Themes, Use of Alcohol, Language
GTA V is also M 17+, but with Intense Violence Blood and Gore Nudity Mature Humor Strong Language Strong Sexual Content Use of Drugs and Alcohol
FF 7 remake is T (13+), with Violence Blood Mild Suggestive Themes Language Use of Alcohol and Tobacco
Since there is no nudity in Skyrim it might get away without the nsfw marking on Itch. GTA V would not. And FF7 would not even be near the nsfw category. Point is, nsfw category is not strictly the same as 18+.