Trinity
The tunnels shown in that report are the existing Heathrow Express & London Underground routes.
The future plans for T2 fit into that overall masterplan - future additional satellite to the East, and others to the West. These will all eventually have underground transit connections.
I imagine that all of these plans will now be put on hold until things pick up again.
As for the demolition of T1, I understand that empty buildings attract Business Rates, whether in use or not, along with maintenance costs. I've seen this within industrial parks where empty buildings are demolished, and the sites left empty until a new client wants something new built.