This is just as true for 3v3 as it is for 4v4 and 5v5.
You’re not going to beat that Thornton line at possession. They’ve been one of the best possession lines in the league for many years now. That is exactly what they are known for. If the Matthews line was to be shut down and hemmed in, that was exactly the line that would do it. Not a great use of your highest skill players, playing in the defensive zone, is it?
The Kadri line is not exactly a dump and chase, grinding line. It has speed, possession and stick handling too, which is why they are a shut down unit that can score. There’s a reason Kadri is called Nifty Mittens, even if he’s been dry for a streak which he broke out of today.
Eventually, whatever line that gets put on first will have to come off the ice and someone will go on. Eventually Thornton had to come off and Matthews had to go on. When putting Matthews on, you’re trying to optimise the amount of time he’s going to be spending in the offensive zone. Putting him against Couture means maybe he’ll get 1:45 of ice time total, 0:50 of which in the offensive zone, whereas if he was stacked up against Thornton he might get 2:00 of ice time total, of which 0:45 are in the offensive zone.
You see, hockey is not as black and white as “if you play defence in 4v4, you’re wrong”, a professional knows that which is why you are not. And is probably why stronger thinkers of the game take exception to your over-simplistic yet disproportionately arrogant take on your own opinion.