This is exactly it. If HFR wasn't feasible, then HSR isn't, so that doesn't make sense.
It probably turned out that HFR would cost say $5 Billion, and HSR cost $40 Billion, and there was a much greater ROI for the latter.
This could be exactly for the reason you hinted at: without straightening the curves on the route through Peterborough to Perth, it probably slowed down the HFR proposal dramatically. Like to the point that it wasn't any faster than the existing trains on the CN line.
Straightening the curves turned out to cost so much, at that point you might as well pay a bit more and make the thing HSR.
It might very well be that through that section they just propose to run the whole line in an elevated guideway that passes over all the geology that caused the curvy route in the first place, and you get full grade separation over roads to boot.
Once you do that, you basically already have a formula for HSR.