You need to maintain final reserve fuel + alternate fuel at all times during flight.
Unless landing is assured at destination.
In this example landing was most certainly assured with regards to normal factors (weather, technical status, etc). So this was completely the correct decision.
If they’d diverted then on 9 mile final at the alternate airport they’d have significantly less fuel onboard and be in the exact same situation. Landing at the original destination is the safest and most logical option.