That’s not exactly the case, your second link even states that HS2 would offset (via modal shift) more carbon than it emits during construction.
That ofcourse isn’t to say we should hand wave it away, construction is one of the worst industries when it comes to pollution. But, the problem is we will need to build things, because the structure of society needs to change to stop being so carbon dependant. If we do nothing, then we still have all the structural problems which caused the problem in the first place.
HS2 stops, some, people having to drive. Longer term, it would also help to move business concentration away from the south east. Decentralisation across the entire UK would mean that a lot more things are a lot more local to a lot more people. HS2 doesn’t do that alone, but HS2-type projects (my original comment) absolutely do.
Even if you support degrowth as a method, that still requires substantial change, which in turn means construction.
