I agree that postman has a moat. I think it’s important to say that every company has a moat in that they’re organized, located somewhere, and have customers. The size of the moat varies. We can treat a moat as a scalar value of dollars to cross.
Every company also has a valuation, either through public markets, private markets, or discounted cash flows.
A house cleaning company has a moat of trained cleaners, existing client generation process, goodwill of existing clients, etc. If the value of their discounted cash flows exceed the cost of crossing their moat, they are vulnerable.
The argument is that crossing postman’s moat would cost much less than their private valuation, and their future roadmap is unlikely to build a moat that is significantly harder to cross.