How do I restore a Deja Dup backup (done in Ubuntu) in Mountain Lion?
Deja dup uses duplicity for storage. You may want to use duplicity in the command line, which is available for macOS.

Install it with Homebrew like this (install Homebrew first if you don't have it yet):

brew install duplicity

Then restore with something like:

duplicity --file-to-restore the/file/you/want/to/restore file:///backup/location /where/you/want/to/restore

Not sure if this will help OP, but could help other people.