I discovered rsnapshot hadn't run in a few days. Checking /etc/rsnapshot.log, I found every recent day had this:

rsync: --delete does not work without --recursive (-r) or --dirs (-d).
rsync error: syntax or usage error (code 1) at main.c(1795) [client=3.2.7]

A few days ago I had added the line rsync_short_args -W to /etc/rsnapshot.conf in an effort to get rsync to run without putting such a load on my system. Removing this and running rsnapshot -v hourly from the command line shows that without it, the first line of the rsync command was /usr/bin/rsync -ax --delete --numeric-ids --relative --delete-excluded \, but with it, the first line was /usr/bin/rsync -Wx --delete --numeric-ids --relative --delete-excluded \.

Changing the line rsync_short_args -W to rsync_short_args -Wa, with an a flag explicitly included, solved the problem. Apparently specifying custom short flags overrides at least one of the default flags.

Also: remember, when you run an rsnapshot test, you might need to manually remove a partially-begun new backup (hourly.0, in my case) and rename suffix on the backup now called something like hourly.1 back to .0. Otherwise, you might wind up with huge backups eating your diskspace, as rsnapshot uses a technique called hardlinks that depends on adjacent backups being as similar as possible to each other to conserve disk space.