I am not assuming that (I've been in that exact situation: working on a team that makes products for many customers at once, I usually was working on change requests for different customers at the same time).

I think the confusion - that a few others have astutely pointed out - is that some people associate waterfall with a kind of "get the requirements, then change or respond to nothing until the project is done 6 months from now" mindset, which isn't what it is (though it has been done that way in the past).

Waterfall never stopped people from splitting projects into small pieces - small waterfalls, you could say - but in the past it was much harder to do in software, as the tools to make this easier were not as mature as they are now. All that Agile adds to that equation is unnecessary and harmful ideas - it's just modern Waterfall with a bunch of nonsense added to it.