If you scroll down on your
source's source a bit, you'll find another section which distinguishes procs from spells as having a different cap of 400. From that source:
"This affects not just stuns, but also slows and snares and presumably any of the effects with hate that scales with NPC hitpoints. The scaling spell hate portion of a proc is capped at 400-- any damage component will stack. I used a swarmcaller on a 3600 hp NPC and the proc did ~236 hate. The proc on a 11.5k hp NPC did 400 hate."
I used Sentry Enots to parse hate, as he has 18k HP, this would imply a 18k/15=1200 cap from the spell hate method. WESS, Willsapper, Nev's Horn, Tserrina's Whip, Silken Whip of Ensnaring, Glaive of Marltek and Trochilic's Skean all parsed around 400-450. I strongly suspect the cap for most non-damaging components is 400.
Two weapons confound that, Ykesha (730-814) and Kunzar Kujuch (626-710), which parse higher for some reason. Still trying to figure out why, maybe I pushed them out of melee range or something.