It's correct, just not in the way that you'd think :P
THM is one of the classes required to get Summoner. If you click the Austerities of Flame quest, you'll see that it lists Thaumaturge being required as level 15 to get it. Arcanist 30, THM 15.. So technically it's a THM quest! :D
(Yes, it's a silly explanation on a silly technicality, but that's how our system lists it ><)