can stand frost down to -18 C.

That's 0 in Fahrenheit. Pretty cold. We would consider that USDA Zone 7, so you
are getting conflicting information. Zone 5 means an average minimum winter
temperature of -10 to -20 F, or about -23 to -28 C.

So what the hell do you call "fully hardy"? That means neither the top
nor the roots will die during an average winter in that zone. I think what your
references mean is that the yam is fully hardy in Zone 7, but in Zone 5, the
top will die over the winter, and the roots will survive with a little added
protection. In Temperate zones, it often helps to find out where the frost line
is. Here in Zone 5 in Central NY, the average frost depth is about 36 inches (I
think), but it can vary depending on your soil type.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"When you come to a fork in the road, take it." Yogi Berra