No, Twitter isn't private, but neither is it forcing anyone to read it. Those two guys forced the people around them to listen to a lewd joke, whether or not they intended to.
There's also a matter of context. A personal twitter stream is a very different context than a professional event. If I'm friends with my coworkers, I may hang out with them after work, and a dirty joke then is perfectly acceptable. But with the exact same people, at the workplace, a dirty joke is inappropriate. Exact same people. The difference is context.
Well, yes and no. If you're the kind of person who sometimes, in some contexts makes lewd jokes and is not offended by them, then simply being in another context when hearing them shouldn't offend you. It might make you cringe at the incongruence but it won't offend.
Furthermore, you wouldn't then see yourself as a "hero" for publicly shaming the people who made said lewd joke. Simply turning around and telling them that what they're doing isn't appropriate.
Also, it's not like those two developers were using a microphone or loudspeaker or some such device, they were just chatting away behind her and talking to each other. Sometimes we forget that people may overhear us when we talk to each other in a public setting. In that context is inappropriate speech(not sexist, not outright wrong, just inappropriate for that context) grounds for public shaming and firing when you simply could've forgotten(or not thought about) being overheard?
I just think that she went overboard with the reaction and even seeing herself as heroic for doing something like that. The fact that she makes jokes like that in a different context simply shows that it couldn't have been that offensive to her(maybe just cringe-worthy) to warrant such a reaction.
There's also a matter of context. A personal twitter stream is a very different context than a professional event. If I'm friends with my coworkers, I may hang out with them after work, and a dirty joke then is perfectly acceptable. But with the exact same people, at the workplace, a dirty joke is inappropriate. Exact same people. The difference is context.