In hindsight, yes, it's basic for sure. But in the heat of the moment after 7 hours of thinking about nothing but chess, Ian obviously missed the potential fork and was visibly surprised that Magnus left his rook to be attacked.
He didn't miss it though - if he had, he would have taken the rook. No one at their level misses something like this in these games. This stuff is obvious to much much weaker players than Ian.
If he was aware of it, he would not have allowed it occur to begin with. He was counting on Magnus to move the rook and was very visibly surprised when Magnus left the rook in place.
Replying to sibling:
no, he was not - it was obvious, even to a low-level recreational player (like me)
Copying what I said in another reply: I'm referring to move 133, right after Ian moved his queen from H6 to H7, attacking the rook. Magnus ignores this and moves his E5 pawn to E6. https://www.youtube.com/watch?v=PWkBeTwbfj4&t=29890s