You don't know he was speaking to VAR, he could have been talking to his assistants and 4th official. I'm pretty sure 4th official, Andy Madley, who had by far the best view of it, has told him it was a foul and to go red.
Would also add having seen the replay that Sky showed, cutting it before the injury, Struijk was definitely off the floor with both feet. His trailing leg has then smashed into Elliott's standing leg, there is a lot of force in that challenge and it is easy to see how the injury has happened. Red was the correct decision, however they managed to come to it.