It’s not about tech people and meetups in particular. Or dresses.
I used to go to startup (business) meetups, no dresses or makeup, regular jeans. It was worse. Everybody headed directly to my husband (who came as a guest), plain ignoring me sitting right next to him, to an extent of being rude. A guy refused to give me his business card. Don’t be fooled when it’s called “subtle” and “subconscious”.
A person who’s got a gendered statement at least has an option of going sarcastic. An invisible ostracised person has no options.