I definitely agree with the sentiment, though I've noticed that a lot of the small presses with discords that I've encountered have basically developed an artistic community of writers via the discord, which could be very productive.
On the whole, I compare them favorably to the discords for open source projects, basically because while open source had previously had a good, accessible ecosystem for project communities (irc+listservs+forums+bug trackers, etc), having a small press develop a community (versus being established to serve an existing scene) seems to be a new development. What's more, people are using discord features in an interesting way (Tenebrous uses voice chat to do pitch party / brainstorming events, for instance).
It's kind of stupid *not* to take email submissions, but I guess it gets rid of the need for a slushpile if you're basically socially vetting every author & getting involved with prospective authors before they even start writing.