Couple problems here; let’s examine.
1-You’re referencing Tom Gulitti. Do better, try harder.
2-Name a Capital who has played well defensively in 2023.
3-The Capitals Blue-line literally featured two guys who don’t belong on an NHL roster (Matt Irwin and Gabriel Carlsson) and a guy who has spent the majority of the season in the AHL (Alexeyev).
4-The Capitals are not good. Good players will carry a Minus for the Capitals for the rest of the season as the Capitals are likely to be outscored at Even Strength for the rest of the season.
5-Sandin’s principle role on the Caps for the foreseeable future is to QB the PP (and learn to be a good distributor to #8 while Carlson is out). Guess where PP goals don’t show up?
Evaluating Sandin by his +/- might suit your narrative but it is incredibly naive and not particularly informative.
Sandin is playing De Facto 1D minutes for the Capitals, who are, again, quite bad. He’ll be a Minus for the rest of the season, he’ll also be a 1D. I’d expect that experience will do him plenty of good when the Caps have a semi-competent D-Core next season and Sandin can transition to playing less than super-human 1D minutes on a Lottery Team (which is precisely where he is today).