Giles was drafted in 2017 but didn't play at all that season due to injury. He actually played 3 seasons:
2018-19 (58 games)
2019-20 (46 games)
2020-21 (38 games)
So in 142 NBA games, he has averaged 5.9 pts, 3.8 rebs, with 51% FG%, 25.8 3P%, in 12.9 min/gm. That is not terribly impressive for an NBA player, but really good for a 2-way player. Grant Williams, for perspective, is a career 6.2 pts / 3.4 Rebs in 288 games with 21 min/gm. The big difference for Grant of course is his 38% 3P% for his career. Grant is 24, Giles is 25.
Maybe the Celtics believe Giles is shot, like Jabari Parker turned out to be, and that may very well be the case. Or maybe based on his 142 NBA games, they just don't feel he is that good in any case. He sure seems like he would be worth a 2-way contact. He petitioned the NBA to get that status that allows him to be eligible for a 2-way contract (essentially his rookie year, where he didn't play a minute, does not count as year played for him). Why do that if you aren't willing to take a 2-way contact?