Ah okay I see it. Yeah it's a mistake, they likely used the same template for the 310 and 410 since they're more or less identical, and forgot to edit that part out.
The obvious route to go for backup power would be a small UPS, which you can get for around $50 and the scanner by itself would probably run for 24 hours on it.
Or you can be creative and make use of something you probably already have. If you have a cordless drill that uses 12v or 14v batteries, you can run it off those. Just take the DC cord that comes with the scanner and attach some quick-connect terminals to it, which will slip right over the battery's terminals and keep the wire in place.