This news is notable because a mainstream proprietary software publisher has chosen to officially distribute using Flathub. I couldn't care less about Discord, but it says a lot about Flathub's mindshare. Proprietary publishers tend to pick AppImage, but this is the first publisher I know of that has chosen Flathub instead. It will be interesting to see if this becomes a trend.
That being said, Cassidy obviously had a very active hand in convincing Discord to adopt the Flatpak package.