hmm, my download had a godot folder with an autotile.png but no project file (project.godot) or .tres file (which I think can hold bitmask definitions).
As far as I know, with at png I need to use the Godot tilemap tools to manually draw the bitmasks. Again, I'm new to all this so I could easily be missing something obvious. Apologies if that is so. If you get a chance to take a look it would be greatly appreciated. These tilesets are amazing, looking forward to building out a game with them.
cheers