It's not science fiction, it's just drilling holes in the ground vs waiting for flying cars.
It may be expensive - but you only have to build each tunnel once.
There's a startup that got some hype recently for flying an electric airplane between London and Paris - when as someone commented on the HN story [0] you can just take the Eurostar through the tunnel - which is electric and takes just as long and doesn't cost any more.
I'd guess 1 lane at 200km/h without stopping is ~7-10 lanes rush hour traffic.
In traffic, you have to look at both throughput and carrying capacity. Traffic jams usually result when the carrying capacity gets strained, not throughput. (The throughput of stop-and-go traffic is lower than the throughput of 70 mph traffic, but the road can carry more cars in total, so that equilibrium dominates when the load coming from surface streets exceeds some threshold.)
It would be interesting I guess if they combined it with the Hyperloop concept, board the tube system in the city, transition to a (cheaper?) above-ground vacuum tunnel system at a transit station.
But it's all very science-fiction and, if anything, it's not going to be cheap. Like, at all.
At least it's a commercial project, not something a government has to pay for.