I’m reading the Caverns of Thracia and I see the maps have a scale of 1” to 40’ for the most part - that seems to be about 4 squares.
My understanding is that in AD&D if you have a movement rate of 12”, that means you move 120’ in “imaginary world scale”.
Should I be using a scale of 1” = 10’ on the table (assuming I’m using miniatures), or do my PCs move at 480’/round on the map as drawn in the module?
Always use the scale in the rules when translating movement statistics to distance in the game world.
Maps can be printed at arbitrary scales, and map scales aren’t relevant to that rule. It wouldn’t make sense to let characters move in inches on an arbitrary scale: that would make them move faster or slower depending on whether the map was zoomed in or out when printed!
Also note that dungeon maps were never designed for use as battle maps with miniatures. The design of AD&D era maps revolves around conveying information to the DM so that they can describe the place being explored and adjudicate PC actions.
A map scale of 1 square = 10′ is extremely common, but it’s only because ¼″ quadrille paper is very common in the US, and letting each square be 10 feet a side gives enough room to draft an overhead schematic of a reasonably sized adventure location.