I looked at the code to see why it would be doing that. It seems to handle the font customization stuff by basically downloading all of the variants and then combining them into a zip file in client-side code - even when you do no customization at all. Apparently that code which makes the zip (which I'm guessing is an external library) creates a corrupted one when run in Safari according to a comment buried in the JS.
Maybe it really is a bug on Safari's part but creating custom zip archives is something which would be far saner to do on the server side in the first place.