Isn't that the core functionality that they say they won't charge for?

If not, I don't understand why they bothered mentioning "how will we make money?" on their website, and then why they didn't answer with, "By charging game vendors a ton of money for the SDK!"

What's the point? Tons of game engines already offer that in-engine, eliminating the need for Discord's ability to do that. (It's been around since UT2K3 earliest I can remember.)

Stickers are more than something to sell to consumers - They can be advertisements used to target a gaming audience. Skype and Facebook have done this, offering free sticker packs for upcoming movies (e.g. Inside Out). A pack of stickers or a theme featuring characters from a game like Overwatch could potentially bring in revenue from both Blizzard and players.