The design of email means there is an argument for the legal system that gmail is a very very popular post office. That email is a system wherein users have disparate storage areas for receiving/archiving emails, and gmail is one of many organizations that offers users the service of managing their individualized storage area on their behalf.
In the service that is email, there is nothing inherently special about gmail. In the service that is discord, discord is discord.
IRC enables Text & Attachments in text form (ie links)
Email Enables Attachments in visual (html emails)
Discord Enables Voice, Text and Attachments (Asynchronous sockets)
That's all it is. Present a Video, Picture, Voice to your visitor and you've got a new product. Just as facebook,twitter,myspace.
Assuming all this is correct, if a GMail user requests their account to be deleted, Google should delete that data.
On discord there are no 200 individual "they" storing it. It's stored by discord. (Or I guess people could be manually logging their own chats, but that's not the issue at hand here.)
At first pass it might sound like some minor technical squabble about implementation details, but there is intentionality in design decisions. The legal system can not be blind to those.
Discord chose a design that put them in control of the data. I'm not saying the highly centralized nature of their design is bad, wrong, or wasn't the best choice for the product they want to build. What I am saying is that when the legal system looks at the nature of a product, it has to look beyond what's presented in the pretty UI and marketing materials. It will look at the business' processes too.