As I say to passengers, it is better for us to have TOO MUCH information about disabled/pax requiring assistance than not enough.
Unfortunately in this case it seems to be misunderstanding on both sides. Mrs Hurst, being acquainted with the parties involved, may well be of the opinion that they are perfectly able to find their way around etc (as most blind people are). But in the context of the airline they are 'disabled'. Perhaps on being asked if they were 'disabled' her interpretation of disabled differed from theirs. It's the classic 'hear-back' - you hear what you want to hear.
If the rules say only four per aircraft then it is sad for those offloaded but rules are rules. I do think though that the pax could have been treated with a bit more compassion and sympathy (if the article is accurate). Sleeping on the floor is unacceptable if the airline has offloaded them for reasons beyond the pax's control. Regardless of whther it was Ryanair who stuffed the booking or the carer, I would be trying my darnedest to prevent my pax from having to sleep on a floor.
As to evacuation, true that blind people would likely be able to find their way around in a smoky cabin, but add to that everyone else also trying to get out, panic, fire etc, and also in the event of a crash the cabin layout is going to be somewhat rearranged!!! Not to mention the issue of whether an exit is safe to use, and falling off slides etc...
I think though that this will make them review their procedure... they'd be crazy not to!!
I must admit I have had problems with pax who make bookings over the internet and negelct to read the part about pax requiring assistance to notify the call centre prior to flying with us (and not a half hour before!!) The amount who show up who have just been in hospital and not even asked for a Fit to Fly is staggering!!! Of course it's all OUR fault...
what I'm saying here is that the airlines do the best they can with the information px give them. If you are in ANY way unsure, ASK!!!! It can't hurt and could very well save you the situation mentioned in the original post...