That doesn't limit in any way the expression of the belief that there are only two genders. Feel free to run a Twitter search and confirm that for yourself (I just did). I have to say I am genuinely curious to find a conservative belief that I wouldn't find thousands of (unironic/uncritical) hits for in a matter of seconds.

It does, you can't tell a non-binary person that they're a man or a woman, if your tweet is reported it will be deleted. If you can find any examples of this they simply haven't been reported or reviewed yet.

Intentional misgendering of an individual could be sexual harassment; it is certainly not a form of "conservative speech" regardless of beliefs. However, the belief that there only two genders is not only not banned by Twitter but is, in fact, widely disseminated.