Here, it doesn’t mention Height_label as nominal variable. But we can not subtract them to tell the size of the difference then why it isn’t nominal variable?
Because we can tell the direction of the difference even if we can’t tell the size of the difference.
We can say that someone with a
medium label is taller than someone with a
short label. That’s the direction of the difference in this context.
The next Step goes into detail on this as well.