Yes, really. Adopted from French from Late Latin thea, tea + -form, but I can't at the moment dig up when. It can also mean having the form of the tea plant, but that seems to be a less-used sense -- insofar as this is still used today. And to be honest, I can think of only one possible use:
Arthur Dent stared into the cup at a liquid that was almost, but not quite, entirely anti-theiform.
Related trivia: caffeine was first extracted from coffee by a German chemist, who named it that from German kaffee, coffee. A little later, a similar substance was extracted from tea by a french chemist, who called it theine from French thea. It was only later that the two were proved to be identical. But think of the possible alternate histories ...