Where would you propose the tunnel start, and the tunnel ends? It's a worthy debate.I don't think he's saying the whole thing has to be a tunnel. Just the station at Pearson has to be underground and hence you would need to tunnel a section of it.
If the tunnel is short, those are rather sharp curves away from the existing surface rail corridor that forces a high speed train to slow down significantly. Best to not bother building the tunnel, if it is going to slow down the HSR train. Waste of money if the tunnel causes the HSR train to arrive later, no? You can make up the speed difference using a high-performance fast LINK upgrade, versus a tunnel curve forcing a HSR train to go slow. Look at how straight the railroad is north of Pearson. A high speed train takes 15 kilometers to accelerate to 300kph, and that's a straight line.