It's not just sustrans that do this. I regularly hear politicians congratulate themselves of X number of miles of cycling routes added. Makes me cringe.
They're the fucking reason a charity is responsible for the cycling network. Imagine if the highways agency had been left to evolve from a group of volunteers forming a charity after tarmacing a local unused farm track.
^ & ^^ yes, when I worked for them I was mystified by the quantity over quality strategy, although iirc this was funding driven.