It's no directory listing, because the files named in it don't exist, save for the most recent one. Why is there no public repository of older snapshot tarballs (in particular, the most recent well-defined version would be of >interest)?
There is no repository of old tar-balls because they aren't saved. They are volatile pieces of source that should only be used for testing. If you must retrieve an old snapshot you can use the date tag shown in Pikefarm.
It helps if there is an official upstream tarball that you can compare the .orig.tar.gz from Debian to, but it's perhaps not that important in experimental.
The test snapshots really shouldn't be used to make Debian packages. They should use a proper make export build.