Of course you can’t really measure on an atomic scale anyway because you can’t decide which atoms are part of the coast and which are floating in the sea. The fuzziness of the “coastline” definition makes measurement meaningless on scales even larger than single atoms and molecules, probably. So you’re right, and we can’t measure it arbitrarily large. It’s just wordplay at that point.
Of course you can’t really measure on an atomic scale anyway because you can’t decide which atoms are part of the coast and which are floating in the sea. The fuzziness of the “coastline” definition makes measurement meaningless on scales even larger than single atoms and molecules, probably. So you’re right, and we can’t measure it arbitrarily large. It’s just wordplay at that point.