That's a promise that Lightning makes and that you can check in their code today, but it is not a fundamental guarantee the way that the bitcoin blockchain offers guarantees.
Sure, if you changed the code of lightning then it would work differently. If you changed the code of the L1 chain it could work differently too. How are those guarantees different?
Calling it a promise makes it sound like there's counterparty risk and you're relying on them not changing the code. Changing the lightning code would have no effect on your funds locked into lightning using the old code.
