That’s surprising to me. I remember at the time, NBC Nightly News and PBS Newshour (my family’s news diet in the 90s) did stories about it, and they both definitely mentioned reclaiming city space as one of the benefits.
I think the Big Dig, while it ended up costing several times what it was supposed to, will go down in history as one of the best highway projects of its era. It also proved infrastructure naysayers wrong. A lot of people insist that any highway projects always just induce demand, resulting in even more congestion, but the Big Dig did nothing of the sort. To this day, 30 years on, Boston traffic is still not as bad as it was pre-Big Dig.