It looks like he was given permission to propose to her, but in a certain spot. Based on the video, I bet they told him he couldn’t do it on that stage, but maybe in front of it, hence maybe why the CM directed them to go down those steps.
As a former DL CM who got asked for permission to propose in front of the castle and even on Peter Pan, I can say that this process is regulated. CMs will tell you where exactly you can do it and when to prevent any issues with other guests. It’s very possible that having the guests be on that stage was a safety hazard and the CM had to get them off of it as soon as possible for both their own safety and for their job security.