They're talking about window units, not heat pump systems or central air. It's a complete AC that sits in your window. It can't cool a very large space (even in apartments people may have more than one), but it's cheap and easy to install (and equally easy to remove). It might be unsightly on a house, but a lot of that is probably just cultural norms. They're very common in NYC, where a lot of housing was built before central air became the default.