If people were using it as a wheelchair, as it's being said it was in the information released, in great enough numbers it wouldn't have been enough to tell one person as you see it to not use it that way.
In the video one of the women in it state she was given a red tag after signing paperwork with the red tag saying a rollator that has a seat can't be used as a wheelchair and failure to adhere to this can mean you would be removed from the park. They have seemingly been on the watch for this issue.
My random guess is there may have been a discussion regarding concerns over liability risks, exactly what Simba's Mom was also thinking and it was just determined that it really reduces their risk by just not having such devices in their parks where the temptation to use them improperly would be there.
Disney has done similar things like when they banned the wagons and reduced the stroller sizes (although they aren't as strict as they used to be) where one day it was okay the next it wasn't and a lot of people were unhappy about it, they adjusted so too will Sea World guests.
Their failure here is lack of communication and too much abruptness in a change. They should have given guests adequate heads up and a date in which their policy would be changing (as did Disney with the stroller size change and wagon ban).
Not going to lie though the woman in the video says she can't find a rollator that has both wheels but no seat saying the ones she found were either the tennis ball ones or the ones you don't have wheels and thus have to pick it up. However, I immediately found some online that have 4 wheels. I do think finding an all-terrain one with robust wheels may be the issue as there's a more limited market for that. Ironically one I found for on
Walmart that has no seat but has a storage pouch (couldn't be used as a seat), 3 wheels the reviewer mentioned they used it at SeaWorld (review was from September and the wheels mentioned are all-terrain though not ones with heavy treads). I assume this particular kind would still be allowed.