No, Tiff Files work just fine, but they have no additional Meta-Data to tell the Engine what they are doing. What the Engine then does is assuming that the Tiff is saved with the default settings (diffuse_lowQ). What the CryTiff Plugin in Photoshop does (or at least how it can be best described) is save a Tiff File and…
I don't use Gimp, but the alpha is stored as a separate channel from the three color channels, so it's probably just a Gimp preview mode that makes it look like they're dependent. If you use a format like PNG, the alpha can change the RGB channels. But a format like TIFF keeps it completely separate.