I tried to edit http://wiki2.dovecot.org/procmail but got stuck on the TextCha. Approaching this list instead seems to be a time-tested secondary approach, and I wasn't completely comfortable making these changes without discussion anyway. So here goes.
The SENDMAIL= assignment is not only irrelevant, but actually wrong. The note down in the update section about Ubuntu explains the problem well enough, but it applies to all the sections on the page.
My proposed edit was to simply remove SENDMAIL and SENDMAILFLAGS from all the recipes on the page, and then remove the Ubuntu section altogether, because it would no longer add anything useful to the page.
If somebody in the Dovecot community with edit access to the wiki wants to implement this change, I would be grateful on behalf of the people who try to use Procmail with Dovecot.
-- If this were my real .signature, it would suck less. Well, maybe not.