Hardlinks in backups aren't made against the mailbox store. E.g. I have all of /opt/zimbra on a single mountpoint, and I do
ls -lt /opt/zimbra/backup/sessions/full-20110226.090011.258/accounts/044/39a/04439a86-f8a1-4466-8d12-b7820e09de42/blobs/1/7
(Note: you'll have to use a different path on your system, but similar structure.) I can see that the maximum links to any given message is 4. This is the same as the number of full backups currently residing on my system. (Excluding "ad-hoc" fulls caused by the addition of new accounts.) In other words, these are messages that are in all four full backups; if there were links to the mailbox store, I should see at least one message with 5 links. I checked against several paths just to be sure.
Note that hardlinks may only occur between consecutive backups, though. If you have a zipped backup in the middle, that may cause the next --noZip backup to be done "fresh" with no hardlinks. Also, I highly doubt that it matters, but my crontab has the arguments to zmbackup in a different order:
0 1 * * 6 /opt/zimbra/bin/zmbackup -f -a all --noZip --mail-report
0 1 * * 0-5 /opt/zimbra/bin/zmbackup -i --noZip --mail-report
0 0 * * * /opt/zimbra/bin/zmbackup -del 1m --mail-report