'doveadm expunge' with -A and userdb { driver = passwd }

[Grant]

I'm expunging mail in my crontab like this:

doveadm search -u user1 mailbox INBOX.Trash savedbefore 30d
doveadm search -u user2 mailbox INBOX.Trash savedbefore 30d
doveadm search -u user3 mailbox INBOX.Trash savedbefore 30d
...

I'd rather do this:

doveadm search -A mailbox INBOX.Trash savedbefore 30d

but I'm told not to:

"If the -A option is present, the command will be performed for all
users. Using this option in combination with system users from userdb
{ driver = passwd } is not recommended, because it contains also users
with a lower UID than the one configured with the first_valid_uid
setting."

I'm using userdb { driver = passwd-file } but I'm guessing that's the
same thing.  Am I understanding this correctly?

- Grant