A commitment to pay Bitcoin, that is signed or unsigned, cashable at any time, is an IOU.
It's the same as a signed check, except lightning is passing itself as a network to spend the equivalent of third party checks. Businesses and untrusted parties do not do this, so they settle (cash checks immediately), which Lightning can't handle at scale with users.
If Lightning wasn't really an IOU, then it wouldn't need Bitcoin to settle transactions, now would it?
It's the same as a signed check, except lightning is passing itself as a network to spend the equivalent of third party checks. Businesses and untrusted parties do not do this, so they settle (cash checks immediately), which Lightning can't handle at scale with users.
If Lightning wasn't really an IOU, then it wouldn't need Bitcoin to settle transactions, now would it?