They do have it. They used to publish it. Then someone found a vulnerability in a commit of theirs (6e42e3b1eca73c306db1580719a3a1bfb715f6d8 if you're interested in looking it up in a mirror), exploited it, and took all data Gab ever had.

This current approach is just a deliberate obfuscation in an attempt for it not to repeat. By just providing a zip you can't really look at their changes on top of Mastodon with git log, you have to put in slightly more effort.