The specific argument could be that the sum entered was falsified by the process digitalizing. And any checksum that would have caught that- would be computed after scanning.

FWIW the form I'm referring to didn't have an amount, rather a box checked "transfer entire account".

But sure, IFF you had an instance where an amount transferred was wrong and their various settlement/accounting tallies didn't catch it, and the bank refused to look into and fix it on their own, and you had a reason to believe they used a paper form for that, then yes you would have a good argument that could be used in court!