I was surprised that the website worked so well on my iPhone. While visiting a friend in the hospital after surgery, his dad, who was booked with me into a studio for June 2020, asked if I could get a two bedroom instead so his ex wife and their granddaughter could go. It was easy to modify the reservation.
We're talking Disney IT. The first issue would be any sort of API integration, because I am fairly sure that the API for the booking systems is hot trash. Then there would be additional issues when they try to do anything WITH the booking system.
The app would then be a memory hog on everyone's phone, too.