grammaticality - For those who know (who knows)

Google numbers are notoriously unreliable, to the extent of not meaning anything at all. For starters, different people get different numbers, and indeed the same person will get different numbers depending on the computer, browser or OS they are using.

In this browser right now, I get 8.6 million results for "for those who know" but only 667k for "for those who knows". That's already quite a different picture from the one that has been painted to you. And when I click through to page 30, that number drops down to just 328. That's a well-known workaround for a well-documented problem.

8550000 results vs. 328 is a whole nother story, isn't it? Though of course the 8550000 number is a lie, too. In fact Google does not serve more than 1000 hits for any query, they have an error message specifically saying just that.

Anyway, you shouldn't be using Google in the first place, but rather a professional corpus, such as the Corpus of Contemporary American English (COCA) or the British National Corpus (BNC):

                      COCA    BNC

for those who knows      0      0     
for those who know      47     11 

Needless to say, it is immediately clear to any native speaker why that would be. Those is plural, so the verb should be plural as well.