Nr of cars vs congestion follows an S-curve with a very narrow/steep middle section. Removing a single digit percentage of cars/passengers can mean the difference between 'busy but only a little slow down' and 'complete stand still'. I don't know about the numbers in this specific case, but transport systems work in not very intuitive ways.
While induced demand is definitely a thing, they're not going to start selling more tickets to baseball games because the road to the park isnt quite as congested. Induced demand is because of the effects it has (encourages more spread out development and 1PV usage).
Why would they waste the enormous amount of additional money it would take to construct a normal metro line, when the entire premise of the company is the proposition that it is possible to construct a useful underground transportation system which is much simpler and therefore much less expensive than a normal metro?
It seems an awful solution compared to the worst underground line.
Modern lines, as I said, can transport more passengers than they move per day in a mere minute...
If you add the time to make 3.6 miles probably they will complete the journey in 10-15 minutes.
Don't underestimate that if those 5% instead of taking the car take this Loop, there'll be less cars and less traffic. So a higher % will be home faster.