A play ends and the ball is dead when a player in possession of the ball is tackled or gives themselves up. Taking a knee isn't some magical thing in the rulebook, it's just an explicit sign the player is giving themselves up; by your logic, a player who takes a knee then stands up and tosses the ball to the official has also put the ball into play. But players give themselves up when they field a kick in the end zone as a matter of course now that the kickoff is moved up and a touchback is more valuable, the scenario here happens multiple times in every NFL game.