No shampoo doesn't mean you don't wash at all, lol. It probably does depend on hair type and skin condition though. Personally, I only shampoo twice a week max and have been doing so since I was in college, it's not a recent fad or anything. (I also use soap/bodywash very sparingly, basically only if I've been sweating or in the rain or whatever.)

I have very long hair that's normally straight (more or less) but goes POOF in humidity, and also very bad/dry skin. I can't claim to understand the science behind it if there is any, so this is just anecdotal evidence at best, but using dandruff shampoo when I was a kid always made my skin condition worse. Lessening shampoo/soap usage was the only thing that helped. Exfoliating is pretty much all that's really necessary for the "clean" feeling, and I also stopped getting split ends as much after cutting out silicone-based products (tbh still don't know if that' s pseudoscience or not, but it worked for me so shrug).

In fact, when I'm in a humid climate I just wash with conditioner instead. Cleans just as effectively and doesn't feel particularly greasy if you're just using a cheap conditioner instead of fancy shit, and really goes to show that shampoo is probably not as necessary as people think it is. (Look into "co-washing," it was and probably still is a thing on curly hair forums for a while.)