Impassable capes are strategically placed in areas to prevent nonocean-going vessels from rounding the Cape of Good Hope or from early North American settlements southwards to Central and South America. I suppose it is possible for Vikings and Aztecs to meet up. But that would require both Vikings and Aztecs to actively send units to Vinland. Very unlikely for AIs to do.
I suppose the only other case in which this may be an issue is if cities placed in cape locations grow to have >500 culture. But that seems very likely as well and in which case the player should prolly deserve some reward bigger than simply meeting another civ