is that even a thing ? any foreigner can be politician in HK ?
Yes and No!
If you are a foreign national, and you don't have the right of abode (identifying having obtained an ID card)! then you can't!
However, if you have obtained an ID card, then you are recogised as a Hong Kong citzens (and for clarity, you can also become a Chinese national overseas CNO, exactly the same as the infamous British national overseas BNO). As such, you can still keep your old passport and national status!
So after you get your ID card, you can vote, you can stand for elections and still be a foreign national.
To obtained a HONG Kong ID card, you only need to have lived in Hong Kong for 7 years! (also, I hear they can fast track you in special circumstances).