The Imperial All Terrain Armored Transport has been abbreviated as AT-AT since it's inception. Whether or not you read that abbreviation as two words like @ @, or say the letters individually like A-T-A-T has been as big of a debate as brown coat versus blue coat. It seems the fact that everyone says A-T-S-T has been the prevailing argument for saying the letters, but this is dead wrong. Commonality of the things being abbreviated has nothing to do with it. All that matters is if the abbreviation can be pronounced. AT-AT can be pronounced as two words, so we do. AT-ST cannot, so we don't.
Want real world proof? The Food and Drug Administration is abbreviated FDA. Housing and Urban Development is abbreviated HUD. They share the commonality that they are both organizations within the United States government, but that has nothing to do with whether or not the abbreviations are pronounced or initialized. FDA cannot be pronounced so we say the initialism F-D-A. HUD can be pronounced, so we say it as a word. See? So it's AT-AT and A-T-S-T and there is no contradiction, but don't take my word for it.
Kenner established the AT-AT pronunciation in 1980. Of course, C-3PO says it like it's a character name, but he's a well known idiot:
One last thing, if there is any way that Disney could collect a bunch restored Kenner Star Wars commercials and stream them on Disney+, I would watch them more frequently and eagerly than some of the big budget movies on the platform.