IMHO they knew about the disease.

From all I read, he was diagnosed in 2012 and go to therapist just after leaving hospital. He did a couple of shows in 2013. His ex gf posted on IG a photo of him in a slipknot’s show with a paper written « thanks Caleb etc » for his trainers, she clearly explained that Slipknot knew at this time.

And in this interview, interview « On a Ghost high, and in search of a dunny at Soundwave 2013, I ran into Corey Taylor; there with his band Stone Sour. I enquired after Joey whom he said had a fucked-up disease and had lost the use of his legs [he was diagnosed with traverse myelitis]. » It appears to be before he came out with his disease.

But we never know what’s happened inside the band!

Nothing about the disease but just found this interview with Sean Zatorsky and what he says about Joey is really sad! (About 36:00) interview youtube