I wouldn't consider any platforms distributing solely their own titles as a Steam competitor. Aka, Origin, Battle.net, etc. are not Steam competitors. GOG and Humble Bundle are Steam competitors, and I wouldn't argue either of them are on the same scale as Steam. And Discord needs to make a lot of money if they're going to pay back their investors.
I stand corrected, then. However, the fact that I was wholly unaware of this also suggests they are not really a viable/successful Steam competitor. ;)