I'm certain you're correct. I've seen many SQL injection attacks, and not one of them has ever labelled itself as such.
I think this shows a defect in the blind voting we've had here for the last year or so. There's no way this off-hand comment is worth that much karma, but nobody can see that I'm being overcompensated for it.
(Sorry for the OT meta-post)
'; DO $$ BEGIN RAISE NOTICE 'Commencing SQLI.'; END $$; -- Use acronym "SQLI" for stealth reasons
Edit: That's probably what it is. A port scanner climbed through port 9100 and hit the JetDirect port on the printer, which prints whatever raw data it is given. Cool find!