Is it ok to use 他 as a gender neutral personal pronoun (instead of 她) when the subject is known to be female?
First of all, there is only 他 in ancient Chinese. The invention of the font 她 comes from the influence of foreign languages in 1900s.

And there is no word in ancient Chinese contains the usage of IT as a gender neutral personal pronoun. Consulting foreign languages we borrow 它 as IT. 它 is actually the ancient word of 他. 它 means "other" in common use.

Therefore, when you use 他/她/它, try to think about it in your language and that's all clearly. 

But someone uses 他 instead of 她 online, that's because they type too quickly to select the word...