Question about Epicenter and CC

Forgive my noobness here, I only recently started playing DotA and am trying to learn the basics of as many characters as possible!

So the other day I was playing Sand King and had the below happen-
Play Video
I showed a friend who has played far more DotA (& LoL) than me and he says this is a bug, as Hex acts as a silence and also a purge effect of sorts. My limited understanding of the mechanic is that once the 2sec channel is complete the spell is there to stay, just the same as silencing a Hero right after they launch a projectile isn't going to suddenly make the projectile disappear. I've searched Google to see if this has come up before but I couldn't find anything, and the DotA 2 wiki doesn't mention anything about the effects of CC once the channel is complete.

So, working as intended or bug?

Working as intended afaik, once the 2 second channel on the ult is done there aren't many things that can stop it.
In dota1 Eul's Scepter of Divinity used to be a popular way to cancel Sand King's ultimate even after he channeled it (the effect stopped once you cycloned him, don't know if bug or intended but it's been like that for ages, i did not test this in dota2 yet)

*edit*
k, i tested this in dota2 & it seems to no longer work, oh well

Working as intended, not a bug.

Excellent, although I imagine the chances of having it happen again are slim :)

very cool.

Its supposed to happen. With Sand King's Epicenter after the channelling is finished, nothing can stop it. There are many times when Sand King dies while using Epicenter, but the ulti still continues. For the best IMO, if they took that out SK would be nerfed to the max then...