The principle that CBS employs is teamwork-oriented. If he can force opponents to score one-on-one and limit threes, and if the Celtics are scoring as a team instead of one-on-one, then the Celtics are more likely to win.
This especially works against teams that only have one guy that can go one-on-one at a high level. Let Giannis score 35, and then force the other Bucks players to create for themselves, instead of being able to play off Giannis. Every time Brogdon, Bledsoe, Lopez, Divencenzo, Snell, Dellavadova, or Illyasova (and to a lesser extent, Middleton) try to score one-on-one against the length and IQ of the Celtics, it is a good defensive possession. This defense doesn't look great when the other team is making improbable shots, but it's the only kind of defense teams can play in the modern NBA.
Obviously, this gets tougher when the other team has multiple players that can score one-on-one at a high level.
CBS thinks Brown, Smart, Morris, Horford, and Semi (and also Williams) can defend one-on-one without fouling. He puts them on an Island to defend the other team's best player, encourages the opposition to stagnate their offense and go one-on-one, and then lives with the results.