You have to mark your game yourself. If you let staff do it, chances are you will be banned or delisted, as not marking it, if you were required to, is a tos violation.
If you are in doubt, look at any similar game that is on Steam. They write age restrictions there. Just guess what your game would get as a classification.
That being said, what constitutes nsfw is quite arbitrary and inconsistent in translation to an age rating and vice versa. This goes as well for the wording used in the faq "strong adult themes". It is debateable what this is or where the line is. And to make it complicated, worldwide standards are different. Itch is us based and so is the nsfw term. Boobs will get you far quicker in that territory, than pixel blood.
But unless you do a comedy zombie game like plants vs zombies, chances are your game is not aimed at teenagers, so why do you even need to ask? Just mark your gore game.