It's really just this line that's confusing, right? Because n isn't really mandatory.

A reader:

• MUST fail the payment if any mandatory field (p, h, s, n) does not have the correct length (52, 52, 52, 53).

Because later it's more clear with:

A reader:
...

• if a valid n field is provided:
• MUST use the n field to validate the signature instead of performing public-key recovery.
• If the signature is not compliant with the low-S standard rule^low-S:
  • MUST fail the payment

If it confused you, I'm sure it's confused others. I think it's worthwhile to fix unclear wording in the spec.