It's not seven times longer. The Austrian line is 130km with 50km of tunnels.
Length Tunnels Bridges Stations Cost
Koralm 130km ~50km 100 12 €6b
HS2 230km ~75km 100+ 4 €74b+
Obviously this does not give any indication of the complexity of each project. Tunnelling and building railway through a metropolis I would imagine is quite challenging.