your unconcious bias is at work my friend, we are in the woke aqaurian age: Felicity could be a man looking for a hot boyfriend! gender identity is fluid - please dont box people into your preconceptions!
If someone identify as a woman regardless of her genitals, then it would be rude not to call her a woman, especially on the internet where she can live her gender fully without the stress of conscious bias like physical clues.
In this case assuming she's a woman is absolutely more likely than the opposite, but there are many cases where blindly assuming someone is representative for a majority ends up erasing minorities.