"But Uli Ernst, a behavioral ecologist at the the Apicultural State Institute at the University of Hohenheim, Germany, adds that
since older and younger ferns (their clones) live together sharing water and nutrients, one could technically call these overlapping generations and brood care." (emphasis mine)
In the same paragraph, clones, not the same plant. Strawberries clone themselves. They aren't the same plant.
"The difference, Burns says, is that the whole strawberry patch looks the same. The ferns, by contrast, differ markedly in morphology and reproduction depending on their role in the colony." (emphasis mine)
Right underneath the paragraph you were quoting, they have different roles, not just different heights.
Also,
"Drawing conclusions about staghorn social organization may ultimately hinge on the nuances of eusociality—some definitions frame the concept as more of a spectrum, notes evolutionary biologist Guy Cooper, at the University of Oxford in the UK, who was not involved in the fern study." (emphasis mine)
So I guess its significance depends on how you define "eusocial".