What upset people is that if it had been a good looking guy people would not have assumed he couldn't be an engineer.
You seriously think this is true? Really and actually?
The stereotype is *absolutely* that engineers are not good-looking people of any gender. A good looking Calvin Klein-style (or wherever it is the hot guys are nowadays) man would *absolutely* take flack over whether he was a real engineer or just a model. Frankly the assumption is that people in any of those kinds of ads *are* models and it's sort of a surprise if *anyone* in an ad isn't (and even when they say "real customer" in a commercial I'm not sure I believe it).
Look, I'm really sympathetic if you are an engineer and happen to be female and at a conference people assume you're a recruiter or something. Assumptions suck when they're wrong, and I've been there. The one about being assumed to be the waiter and given an order is an old joke. But without assumptions about people the world doesn't work.
I dare you to avoid assuming *anything* about the next person you meet at work. Start with "do you speak English/native language", then which pronoun (he/she/they/it/xe) they prefer, then where they work (maybe they're visiting!), etc. These and thousands more are assumptions you make all day every day. The alternative is utter insanity.
Is it a problem? Sure, sometimes you mess up - like thinking someone works at a store when they don't - and it's awkward. You fix your assumption and move on. If you don't actually change your mental model right away, then there's a problem - with the woman-at-conference-who's-a-developer example, you'd better not avoid asking her technical questions or asking a less senior male coworker instead, etc, as that is like the definition of sexism. It does happen and those people are 100% part of the problem.
But we're trying to make certain assumptions not acceptable, even if they are highly likely to be accurate. Good luck with that. A buddy of mine in college was - to put it mildly - a good-looking well-toned guy, and I constantly got asked jokingly-but-half-serious if (with varying levels of obfuscation) he was a dumb jock and I was the nerd he made do his computer homework. It was insulting to both of us, but we both understood that it was a more common situation than the good-looking muscly guy being a brilliant CS major, for that same guy to be hanging out with a nerd like me, and for that nerd to be so desperate to "hang out with the cool kids" to be willing to do someone else's homework. No, no, and no - but we got why people thought it and never got too annoyed unless they kept at it.