Bingo.
There's a natural tendency when you have a lead to make more safe plays regardless of what strategy is implemented, but that shouldn't be the reason for giving up goals the way they often do. What people forget is that there are two teams on the ice competing. As time starts to become more of a factor the opposition can change what they're doing as well. They'll become more desperate and try different things and/or take more risks. For the third period specifically, the players have had 40 minutes of playing against what the Islanders are doing and they might be making adjustments to what has been happening as well.
I didn't notice the Islanders sitting back more, they just didn't have the puck as much as in the first period. Guys like Barzal weren't dictating play as much anymore and it's incredibly unlikely that a coach would tell his players to stop doing that. What's more likely is that Carolina started to adjust to whatever he, and the rest of the team, was doing. Fatigue can also be a thing, even if it's minor. Sitting Bolduc and rolling 5 defenseman out there can create a disruption (and that would definitely be on the coach) in chemistry and flow. You don't get 100 shot attempts in a single period, they were throwing everything at the net from the first drop of the puck.
It's just way more than "the coach is telling them to sit back."