I can see the logic in what they are saying, but it's more related to jurisdiction, rather than sea state. 200 NM is, to my knowledge, the limit of territorial waters and thus the CG's bailiwick. (although I've heard they are taking liberties under the guise of surpressing liberties of late in this regard).
I can buy that coastal is 25-50 NM out (VHF plus an eight-hour run by, say, a 30 footer in modest air), but that coastal has to be considered in the context of islands, barrier or otherwise, shoals, and the continental shelf itself. Some cite the 100 fathom line
as "offshore", by that standard.
Also, coastal Patagonia is a little different than coastal Nova Scotia. Context is everything, as are element like fetch, sloping of the bottom, and current and tidal effects. Sometimes you need a "bluewater" grade of boat to handle some "coastal" waters, which can be very fierce indeed.