Giolito did post a 4.1 WAR season in 2021, 2.0 WAR season in 2020, and a 5.2 WAR season in 2019. Had K/9 rates of 10.13, 12.07 and 11.62 during those three seasons.
At first glance when I saw the deal, yuck. But Red Sox paying an extra couple of dollars for the upside. He's still only 29 and they've done a good job in the past with broken down pitchers and helping turn their career back around e.g. Nathan Eovaldi. Obviously if he has a solid year, he opts out. If he sucks again, stuck having him around a second year. Gotta hope he can produce close to a 2 WAR season worst case.