The article gives ants, termites, and naked mole rats, as eusocial taxa. If these staghorn ferns qualify then I think that siphonophores, such as the Portuguese man-of-war, would also do so.
I see that, in Wikipedia, Eusociality - Wikipedia
there are two references, 10 & 11, to plant eusociality.
Some suggest a criterion for eusociality known as “the point of no return”. This is characterized by eusocial individuals that become fixed into one behavioral group, which usually occurs before reproductive maturity. This prevents them from transitioning between behavioral groups. That’s for animals of course.
These ferns seem to be less than permanent in their castes, but it’s all very interesting.
I haven’t finished reading the Wikipedia article yet, but I agree that Physalia looks a good contender for a eusocial hydroid.
Siphonophores are so tightly integrated that they’ve been considered single organisms, which would exclude than from eusocialdoom. (They’re also clonal.)
Anyway, Wikipedia points me to this report of a giant siphonophore.