We do support playing the Steam version of the game, but as you noticed one of the data file is embedded into the executable, which means we need access to that executable. This is why the folder that contains that executable needs to be used for detection, and not the "DIG" subdolder.
This is indicated in https://wiki.scummvm.org/index.php?titl ... data_files
Note: The Steam version uses a custom launcher with datafiles embedded in the executable. You must add the root of the folder (or Mac app bundle) to ScummVM for the game to be detected correctly, not just the data directory.