I've just opened the OSM site on my PC to have a look at the history in my area, and I've seen this apparently massive edit. It covers most of the US, a lot of Canada, and a significant chunk of Europe and the UK.
wheelmap_visitor says that 'This is a proxy user for all wheelchair tag changes happening on Wheelmap by anonymous users of the website and app Wheelmap.org.' so does this mean that if someone has made edits to areas inside the rectangle at some point recently that they all get included in this edit?
Look at the list of ways and nodes on the left panel. They edited some features on one side of the map, and without commiting the changes, started editing somewhere really far away. Then both changes ended in a single changeset which tries to border around the edited features.
https://wiki.openstreetmap.org/wiki/Changeset