I read the article, it talks about the brain as if it's a washing machine, which it's not either.

Why would it not be relevant? While there are white blood cells in CSF, it's several orders of magnitude less than in blood. Is it enough to clean anything? White blood cells don't pass the brain-blood barrier. So yeah, it's somewhat like a snow-globe for a lot of molecules and cells.

So yeah, how is the fluid cleared?

Also article states, that there's even less blood flow during the sleep cycle, so at the same time, there's even less opportunity to wash away things that can pass through blood-brain barrier away to the kidneys and out.

My background is watching a lot of biology and trying to understand.