This is true of women in STEM the world over, the presumption they couldn't possibly know and so using it as a chance to either flex or white knight.
"Haha Google is FOSS because I can view source in chrome" is absurd. "Microsoft Windows is FOSS because i didn't have to pay to upgrade." No one is going to call a guy on those.
The punchline to the entire joke is the guys telling me to shut up because i'm obviously clueless.