He is airing it out because it's a fair offer, and the fan sentiment to this point has been that the Bruins botched this.
Swayman is in a unique spot because his actual skill level is that of a franchise goalie, but being in a tandem with another Vezina-tier goalie meant he was brought along slowly. He doesn't have the on paper resume of an 8x8 type goalie like Hellebuyck, Saros, or even Sorokin.
In terms of the offer, AAV+length may not be the point of contention. There might be other terms that are important, like a NMC, front-loaded money, or buyout proof signing bonuses. 8x8 with a 10-team NTC and no signing bonuses is a very different contract than 8x8, a NMC, and a large chunk of the salary in signing bonuses.