He has 340000 _followers_, not subscribers. Subscriber counts aren't public but if his follower/subscriber ratio is similar to some of the top streamers he'd probably have around 5-6000 subscribers.

Another WoW streamer like the guy from the article (this ones named Zach) was streaming to 8k+ concurrent viewers for over a year without taking the offer to become a Twitch partner. He turned it on and got some ~3k subscribers in the first week.

He was probably making alright donation income, but suddenly having your monthly income jump ~$12000 a month from just flipping a switch is pretty nuts.