It doesn't matter if the primary defender goes under on a switch, because the help defender is picking up the ball-handler. That's the whole reason switching defenses are great for stopping three-point shooters.
The Knicks played Boston by allowing the Celtics to attack the switch and living with the results inside the arc. That led to some foul trouble for KAT and Brunson throughout the series, but it slowed Boston's offense down and broke their rhythm. What the Pacers showed last night is that whether the Knicks are switching or hedging, a good offense will find ways to attack Brunson and KAT.
All of the breakdowns in the final minute, where Nesmith hit three consecutive threes, came from the players getting stuck on screens without the help defender switching or hedging. Two of those shots came with the primary defending falling from trying to go over the screen.