To me, it didn't come across as deliberately evasive. It came across like tier 1 helpdesk not really understanding how a query fits into their pre-defined categories, and trying to be helpful anyway without really understanding the problem.
The later reply about it being because OCR does use what's below the line and it shouldn't be obscured looks like the ticket was escalated to someone who understood what was really being asked for.
Except that you might be missing the key fact: the letter is just a letter. It’s not a form that requests information from the recipient, and the recipient is not instructed to return it. Scanning it would be pointless, because all of the information on it was printed out by them!
Ooh! I wonder if most of the letters they send out are forms (expected to be returned), but all of their letters are on the same paper-stock, which is pre-printed with the (intended for forms) "don't write below" message.
Evidence in favor of this theory: the "don't write" message is in red text. It's cheaper to do two (or more) passes on one high-volume print run - and then single (B&W) impressions on the smaller runs for each individual letter or form - than it would be to do multiple passes for each and every order.
The support staff wouldn't be privy to these sorts of economic optimizations, so no wonder they couldn't give the guy a comprehensive answer.
The later reply about it being because OCR does use what's below the line and it shouldn't be obscured looks like the ticket was escalated to someone who understood what was really being asked for.
Obligatory xkcd: https://xkcd.com/806/