I'm not sure 'anyone who can hover can long line' is accepted by most of the industry.
I've just finished working on an immigration case about long lining - at least 'production' long lining involving repeated lifts of things like drill rigs or loads from ships where speed is paramount.
The consensus of opinion seems to be that between 3 to 5% of pilots can make good long-line production pilots. The main problem seems to be getting someone trained in the first place.
Another fascinating piece of the rotary wing aviation world!