ULA was selected for large contract by the Dod. This contract amounts to 30+ launches and will take years to complete.
Rocket launches are booked years in advanced. ULA also has many launch contracts 30+ from Amazon. This will again take many years to complete.
So that's why they 'have' costumers, because the selection process has already happened. When you buy ULA you are effectively buying their years long launch backlog.
In addition its really hard to become a Airforce costumer, and once you get there, its very hard to fall out of that group. So even while the next big batch of launches has not yet been selected for by the SpaceForce, its a virtual certainty that ULA will receive a large part of that contract.
