And at a cost of ~$20. That's pretty good.

That cost is the infrastructure cost, amortized over a long time, not counting R&D, maintenance, marketing, ticketing, and various personnel costs, assuming it's near capacity.

Translation, it's going to cost more than $20.

Even if it costs $100 for one way, it's still cheaper than plane, and you save an hour on travel time.

Also consider this is bus-tier people moving, not even train. You just have small individual units moving along one track. With even passenger rail, you have to have set departure times measured in at least 10 minute blocks, something like this could have a departing train every 5 minutes and you could go through a turnstable the same way you get on a subway.

Plus the 2 hours spent at the airport in security, waiting, and getting baggage at the end.