If anything, the relationship will go in the other direction. But it's not significant in practice anyway - as Moscafj said, you should simply buy mealy potatoes, instead of trying to divine which ones could be probably mealy.
The hardness of a potato is mostly related to age, both in the sense of the plant's maturity, and in the sense of length of storage. Young plants have firmer parts with more turgor. So, according to the text you cited, the early-in-the-season potato will be waxier, but it will also be firmer. The other factor is storage time after the tuber has been separated from the plant. The longer it's stored, the more moisture it loses and becomes softer. So, you could touch the potato and feel its firmness, but it won't tell you much about how it cooks up.
This is not to be confused with the language matter where other languages divide potatoes into "hard-boiling" for waxy and "soft-boiling" for mealy. The words there refer to the state of the potato after it has been cooked, and logically, the fluffy cooked potatoes are named "soft". This is a straightforward description of the quality you're looking for, but it's not especially well correlated with a hardness or softness you might feel before cooking.