cat dict.txt | rg "ium$" | uniq | sed -z "s/\n/, /g"

Who knows? Perhaps they thought it would be spelled more similarly to aluminium, ammonium, auditorium, calcium, compendium, cranium, delirium, emporium, encomium, equilibrium, eulogium, exordium, geranium, gymnasium, medium, millennium, odium, omnium, opium, opprobrium, pandemonium, potassium, premium, radium, sodium, tedium, uranium...