They might have gotten launch costs so low that a live test with the possibility of doing it over was cheaper than expensive test runs. Another comment mentioned SpaceX was close to losing some key license. Launching too late was probably going to cost more than the worst-case scenario cost of launching early.
> Starlink is the first krypton propelled spacecraft ever flown.
So they're using non-flight-proven technology (Technology readiness level at most 6)... impressive!