What is Discord Link?
This says it supports Discord Link from the Discord extension, but the Discord extension does not mention Discord Link at all. 
How does this plugin and the Discord plugin know the discord id associated with the steam id?

DiscordLink is a library provided by DiscordExtension that plugins can use to share link data with other plugins. On uMod DiscordAuth and DiscordCore have this implemented this. I'm not sure about other sites / Discord plugins.