As I read it, the politician wasn't campaigning for cancellation.
The gist was that the government would be unable to contain a cost overrun which would then sink other government schemes.
ie they are hostage to HS2 and its project management (with Crossrail and other troublesome rail project history as examples).
The channel tunnel cost 2-3 times the budget, but that only hit the shareholders (mainly the contractors and individual investors, the majority of whom were French).
GW and NW electrification also cost 2-3 times the budget, but NR (ie the government) took the hit, and we are still feeling the consequences.
HS2 is now in that dangerous period where the programme managers are reliant on the contractors delivering close to the budget (with inflation raging).