I wouldn't go so far as to say such an idea is on the level of homeopathy—scientist certainly do induce ("entrain") the brain to certain frequencies of operation using flashing lights. The general principle is sound.

But, I would emphasize, they usually do that by first genetically modifying the creature whose brain they want to manipulate to make some of its neurons into opto-couplers. They wouldn't be doing that if there was an easier way. :)

(It turns out that—even without the genetic modification—some frequencies of flashing light do cause the brain to do certain things. See, for example, https://cosmosmagazine.com/biology/flashing-light-clears-tox.... But the effects of this, from what I understand, aren't the same effect you would get by actually getting the brain to entrain to that frequency; it's maybe entraining the pulse-rate of the brain's glymphatic channels or somesuch, but not synaptic firing.)