I noticed this quite a few too times. --help mentions that there is also --help-full. But --help-full errors out.
Example:
> kopia snapshot restore --help
usage: kopia snapshot restore [<flags>] <sources>
[... a lot of help...]
--help-full Show help for all commands, including hidden
[... more help...]
> kopia snapshot restore --help-full
kopia: error: required argument 'sources' not provided, try --help
> kopia content --help-full
kopia: error: must select a subcommand of 'content', try --help
This happens for a quite a few commands …