Lightning transactions are not based on IOUs.
IOUs can be defaulted upon, while BTC received over Lightning cannot.
Also, you need the cooperation of the other party to redeem IOUs. Lightning transactions don't require cooperation for settlement, they can be settled at any time by any of the parties.
All of this means that the BTC that is exchanged over Lightning is just as valuable as any other BTC, unlike IOUs.
> they aren't money until someone makes an actual USD transaction
That's because your definition of "money" is USD, so your sentence is a meaningless tautology.
It's like saying "raw beans aren't real food until someone cooks them". Uhm, sure, but does that mean raw beans aren't valuable or that you can't easily convert them into food if necessary?
And what's your point even? This point is equally applicable to the first layer.
BTC received over Lightning can easily be exchanged for USD if you want, or (even easier) for BTC in the first layer, so why does this point matter at all?