The basis for that statement is that it's 9km from
any facility that supports flight operations of
any kind.
Most hospitals have helipads. So do many industrial parks, businesses like news organizations, colleges, and even private homes.
On the FAA's B4UFLY app, these are all listed. While we are just outside the five miles for Logan airport, the local newspaper has a registered helipad, there are a couple hospitals four miles away, and the bay three miles away is registered as a seaplane landing area. They don't fly in winter, but the app doesn't know that.
Tailwind - New York City to Boston Seaplane Flights
Canada may not have a similar app: but if you are 8.9km from a lake used by seaplanes, you're shut down. Because the government knows best.