Alex
You missed off the fourth paragraph from the CAA advice i.e.
"This aims to allow cost sharing between friends and colleagues and not to provide an air taxi service to members of the public."
Friends and colleagues does not include punters who are introduced by a search on Wingly or anything like it.
Rules is rules and lawyers is lawyers - lawyers are not in the justice business, they are in the arguement business - I find it instructive that Wingly have thought it necessary to add an insurance in case the airplanes cover refuses to pay out or otherwise denies coverage.
As with professional indemnity coverage, the insurance isn't there to pay compensation to your 'clients' it's really there to pay the lawyers to make ones arguments.
Now here's a thing - do you have to declare the "cost sharing" to the entity from which you hire an airplane? I would have said no with friends but renting to satisfy an arrangement like under Wingly - I think one should have to declare as it is the owner's insurance that will be the first port of call in the event of a claim - I would think that a claim and its scale would be work likely with a Wingly type punter.
Overall I still think it looks more than a little dodgy.