For one thing, the two-legged version of the Imperial Walker is called an AT-ST, and you certainly wouldn’t pronounce that name as if those were words, so it would make sense that the four-legged version would get the same treatment.
For what it’s worth, both sides have reasonable arguments.
Having said that, Without an official canon decision, we have to base the answer on whatever other sources are out there, like, say, for instance, a toy commercial for the officially licensed products, which clearly says it’s “at-at.”
While the exact name of the AT-AT is one of the more harmless fan debates of all-time, it’s certainly the sort of thing that will generate internet comments from people who, for whatever reason, have a passionate view on the topic.