In summary: He stopped washing his hair with any products and started using just cold water, in an effort to help with his dandruff problem. Overall seems to have worked for him. Seemed mildly interesting to me.
Not actually NSFW, but I tagged it because the thumbnail is a close-up of some pretty severe dandruff, and there’s a few seconds of video in the middle where he shows the condition of his scalp before quitting shampoo. Just thought I’d mention, in case that makes you squeamish.
So unlike hand soaps etc, shampoo is not trying to sanitize your hair. Washing is to remove dirt and grease, not necessarily kill microbes.
Yes sebum is beneficial to the scalp. And yes stripping too much sebum from the skin (washing too often) can lead to dryness and irritation and dandruff symptoms.
But dandruff can also be caused by fungus and bacteria, see the “Causes - > Microorganisms” section of the Dandruff Wikipedia article. The fungus mentioned here specifically feeds on sebum, so not washing enough to remove that sebum can cause that fungus to thrive.
So yes washing too much can cause dandruff, but so can not washing enough (I linked this interview with a dermatologist in another comment, they say the same thing here.)
There isn’t a one-size-fits-all answer here. But doctors do recommend washing your hair regularly - how regularly depends on a variety of factors.
Fungus. I hate fungus. I know we need it for antibacterial purposes and for trees to even survive, but did it have to be gross.