1″ of movement stat is always 10′ of dungeon movement, regardless of map scale
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.
Why Dungeon module maps are often 1″:40′
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.