I try not to be all doom and gloom, but this is the exact sort and scale of project the UK needs to be successfully completing to combat climate change. The fact we can't is legitimately terrifying.
It's not, so far as I can tell, saying that the project cannot be done. The Red rating, equivalent to unachievable, is measuring against the goals set for the project, including financial.
So what this is saying (or rather confirming) is that it's not achievable based on the current project goals. Or to put it another way, it's over budget and cannot be delivered within budget.
So yeah, still not great that we can't deliver something like this on budget, but if you look in to how it was originally costed for the business case there are holes in the logic large enough to drive several high speed trains through. Sideways. Hopefully the benefits are large enough that they still go through with it, but I suspect that's largely dependent on which narratives take hold during the run up to the next election and whether the Tories can gaslight people in to thinking that country finances work the same way as personal finances.
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.