So what you're saying is that when you grant someone the right to use your content for a specific purpose, you're actually granting them the right to do whatever the hell they want with it?
Again, not a lawyer, but "in connection with operating and providing the Service" reads like a legally important part of that sentence to me.
"Service" in this context is defined earlier in the document:
>the Discord application (the “App”) or the website located at https://discordapp.com (the "Site"), which are collectively referred to as the “Service,”
IMO you're reading this very creatively if you think that grants them a right to publish a book of your private messages. Rights to use your content are granted specifically "in connection with operating and providing the Service." A book of your messages is not in connection with operating/providing the Discord app or website.
Yes, they could use your images in their advertising, or publish a book of your images, or make a movie out of all your messages and videos, or anything else they wanted to do with, as they call it, "Your Content". That's what a perpetual, transferable, worldwide, royalty-free, sublicensable license to create derivative works is.
And no, all those rights are not required to operate Discord.