I know. But as long as they're not broadcast (in the majority of the cases, they will never be broadcast) these transactions "exist" between the participants of the transaction only (which are not only sender and receiver, but also the intermediate nodes who transmitted them). So while they're not typical IOUs (like tokens or exchange balances, for example) they have some properties of IOUs, as they show the "balance" between the participants of the channels which is independent of the current state of the blockchain (only that it cannot be higher in favour of one of the participants than the channel capacity).

But the Bitcoin blockchain acts as an arbiter. So the risk associated to them isn't comparable with "standard IOUs", but much closer to standard Bitcoin transactions.

In reality, I wouldn't describe them as IOUs but as a "new type of thing", although they share some properties with IOUs, and others not.