Eh, depends on how you look at it. "Public" is a pretty vague term. Something might be public in the sense that anyone can come and view, but it can also be public in the sense that anyone can join and participate, but you still must be a joined member to participate (and that participation can be revoked, etc). Twitter and Reddit are the former, publicly visible communications avenues. Discord is ultimately a private communication channel where anyone can join, but must be joined, to participate.
Unlike Reddit and Twitter, Discord was never meant to be a space where your contributions are intended to be publicly viewable. People forget that while Discord is oftentimes used as a replacement for forums, it is actually the spiritual successor to IRC, AIM, and similar chat services, where the data is typically ephemeral. Message history in these services is still a fairly recent addition, and what we're seeing now is the consequence of that data being available.
Some people think that availability of historical data inherently puts it in the same camp as forums, Reddit, and Twitter, but I don't share that view. Discord data is still intended for present members of the given servers, not the public at large, even if anyone in the public space is able to become a present member. The distinction there is a meaningful one.