As I understand it, they didn’t bump the cache version because:
- the file format did not change, and
- the file names for UUID and MD5-directory-name based cache files would never collide, due to the different lengths and extra dashes.
Further more, if the 2.13 libfontconfig encountered a read-only directory without a UUID file, it would fall back to checking for an MD5 cache file.
Looking at the 2.13.91 source, it looks like it will also be able to reuse existing valid UUID based cache files, but will attempt to remove UUID files from writeable directories.