Yes, but they get that back later on with all the productivity increases.
Or to put it another way - think of how much the government could save in taxes if it built no infrastructure at all. You want electricity? Get a generator. You want roads? Pay a new toll every 1km as you go through various private lands.
You have to (attempt to) quantify those productivity increases. There are an unlimited number of nice things government could spend on, but limited budget. It happens that fiber passed muster. Other things don’t.
(The productivity value of fiber can be estimated with a revenue model based on subscriber fees)
Or to put it another way - think of how much the government could save in taxes if it built no infrastructure at all. You want electricity? Get a generator. You want roads? Pay a new toll every 1km as you go through various private lands.