Everything to do with Demonreach is a retcon it seems though.
In Small Favor, where we are introduced to it, a large group of non-wizards were able to reside there long enough to build a cannery and dock. How? The psychic pressure of the island gave Karrin, Michael and even Thomas horrible nightmares after just a day or so of exposure. Its vibes are strong enough to get it excluded from all maps of Lake Michigan and flight paths into Chicago steer clear of it. That's a pretty strong ward.
I could see it being that Karrin and Michael are more 'sensitive' to such things because of their own experiences, ditto Thomas even more so as a White Vampire. Ordinary folks might not 'feel' the repulsion as strongly, or as fast, that might have enabled the cannery to run for a while before nobody could stand it.
Alternatively, the former cannery might have been a 'cover' for something supernatural trying to do something against the island. But that's just wild-ass-guess speculation.
We just don't have enough information, enough basic data.
Also, it's probably not
just the ward. I'm not sure Alfred even could influence the pilots and other people beyond its domain.
I think somebody, somewhere, has to be deliberately pulling strings in the FAA and other institutions to keep flight paths away and so forth. If it was just the repulsion from the island, tracking radars and onboard instruments would detect the planes leaving their course in that area, and Questions would be asked.
For that matter, the cannery had employees, presumably their employment records and tax returns and so forth are on file somewhere. The island exists and if the cannery was there, that suggests that someone in the mundane world theoretically owns the land (my guess would be the White Council through shells).
Speculation: the island is run/owned/controlled by a coalition of supernatural powers including the White Council, but also including some other heavy players too. That might help explain the long gaps in Warden-dom, it might be that the various power players just can't agree on who should be the man (or woman).