It's a letter from one FCC commissioner, of which there are currently 5. He dissents from the decision the commission as a whole came to. There are a lot of companies on the ground that could benefit from that ~$900 million so a single company replacing Starlink is not necessary. The main concern is if the FCC give Starlink money to reach 100/20 and they don't do it (because there are legitimate technical issues to solve before it's possible for Starlink to supply over half a million people with 100/20), it's wasted money. The FCC didn't think it was doable on that time scale.
Doing some math, currently each satellite launch sends up 22 satellites at around 2.8 Gbps per satellite. For each launch, Starlink adds ~61.6 Gbps of capacity. If we cut that up into 100/20 slices, each launch supports 616 customers at 100/20. To support 650,000 subscribers at 100/20, it would take about 1055 perfect launches.
I don't think the FCC was wrong when they said Starlink could not reach 650,000 people at 100/20 by 2025. There aren't enough days to launch one rocket a day to even try to catch up.