I believe it's been confirmed but I've only seen one post by someone who made a serious attempt to get him through the chests & it didn't work.
Given how chancy event chests are with legendaries, I suspect that it's probably not worthwhile for most people to make the attempt. If you have enough gems to have a reasonable chance of getting him from the event chest, you probably had enough gems to grab him the week he was released.
If you had all the other mythics & were just missing this one last card, going all-in on a Ghulvania event would make sense. Otherwise not so much.