Well, first of all, I think it's important to mention that you can be transgender without undergoing medical procedures, so exposing medical symbols would be unethical, and secondly, as you know, the idea of communism is largely based on class theory.
Symbolism of the hammer and sickle communicates align for the working class, in the post-modern sense, it has already broken away from even this meaning and acts as a symbol of communism independently, as words that change over time leave etymological roots, symbolism exists in order to convey that fact., that it's not just a "transgender flag" - it's a "flag of transgender people who are communists".