When the ball hit Semenyo's knee in the middle, Haaland's right foot was further forward than the goalkeeper - as that only left Thiaw level with him, he was in an offside position. Haaland and Thiaw were grappling with each other, and therefore Haaland was interfering with Thiaw by making an obvious action which clearly impacted on Thiaw's ability to play the ball. Some may argue that Thiaw was never going to get there anyway but a precedent has been set that this doesn't really come into consideration.
It looked messy and took too long, possibly because the semi-automated technology didn't work due to the crowd of players, but it was offside.