If I said "we need public transit as a city" am I assuming that everyone lives in my city or am I simply talking about my own city?

That’s exactly what I would assume, because you’re talking like your city is the default and everyone knows which one you’re talking about.

While you're right. It would be better to say something like "I need it for my country" or "US needs it". Since English doesn't distinguish between inclusive and exclusive "we", it can lead to the conclusion that the commenter supposes people reading their comment are from US as well.

It's pedantic. At best because someone wants to virtue signal by tilting at windmills. At worst It's a bad faith argument being made to isolate someone. In both cases it's shite behavior:

An example would be assailing someone for not liking cookies when they simply said they enjoy cake. This tactic was originally used by trolls and hate groups to splinter larger social groups support structure and/or put people on the backfoot... It's become so commonplace people will do it just because the opportunity presents itself. Because someone else will if they don't, anyway. Might as well get the glory of taking someone down a peg.

It's pathetic. Op made an affirmative statement about something they believed in and was promptly shit on by some cunt who brought nothing meaningful to the table themselves. What's worse is the troll initially was getting nothing but positive reinforcement so they could go and do it again. Are we still enjoying all the polarizing "LOL [insert group] BAD!" It really brings the community together.

You don't need to engage every person doing that shit... but for fucks sake stop upvoting it and reinforcing the behavior.