But that was this persons mistake, I'm still not sure what a "Travel Authorisation" even is outside of COVID checks.

Not checking into a flight, because you wont onto an entirely different website, to complete an entirely different form isn't the airline....

Services for wheelchair are handled by the airport, not the airline, again, I don't see anything in this comment where they sorted out the services that are required prior.

1. They make the online check-in thing ridiculously clear and it's been like that for at least a decade now. They send multiple emails warning people too.

2. Having utilised wheelchairs at airports you need to go very early. The airports don't have enough staff and they basically have to assign you someone for an hour to get you through security and on to the plane. The times I've done it, it's always airport staff and nothing to do with the airline.