The reason he noted Crescentias and pets together is due to the Ignition passive, giving Crescentias huge amounts of Attack and Magic. This nearly guarantees that they have the strongest pets and since DoT pets like mermaids can't stack with others, Crescentia is usually the one who gets to use the pet, and usually the one that gets the additional threat from them. So in that sense, it denies other players from using their pets for extra damage.
Other than that, Crescentia DoTs also completely bypass defenses, and endgame mobs have a ton of defense. Due to this, they are much higher than Rangers and Rogues on the threat list.
Soulmakers usually are at the bottom of the list because they are played as support. Their main heal - Cure - doesn't add threat which usually means the have the least threat out of all the healer classes. I rarely, if at all, see SMs played as DPS. I assume it's pretty low.