Discord is definitely in the same realm of scale as Slack, and probably bigger (they publish different metrics, so its hard to say for sure).
The really impressive thing about Discord's scale is the size of their subscriber pools in the pub-sub model. Discord is slightly different than Slack in the sense that every User on a Server receives every message from every Channel; you don't opt-in to Channels as in Slack, and you can't opt-out (though some channels can be restricted to only certain roles within the Server, this is the minority of Channels).
Some of the largest Discord servers have over 1 million ONLINE users actively receiving messages; this is mostly the official servers for major games, like Fortnite, Minecraft, and League of Legends.
In other words, while the MAU/DAU counts may be within the same order of magnitude, Discord's DAUs are more centralized into larger servers, and also tend to be members of more servers than an average Slack DAU. Its a far harder problem.
The chat rooms are oftentimes unusable, but most of these users only lurk. Nonetheless, think about that scale for a second; when a user sends a message, it is delivered (very quickly!) to a million people. That's insane. Then combine that with insanely good, low latency audio, and best-in-class stability; Discord is a very impressive product, possibly one of the most impressive, and does not get nearly enough credit for what they've accomplished.
For comparison; a "Team" in Microsoft Teams (roughly equivalent to a Discord Server or Slack Workspace) is still limited to 5,000 people.
The really impressive thing about Discord's scale is the size of their subscriber pools in the pub-sub model. Discord is slightly different than Slack in the sense that every User on a Server receives every message from every Channel; you don't opt-in to Channels as in Slack, and you can't opt-out (though some channels can be restricted to only certain roles within the Server, this is the minority of Channels).
Some of the largest Discord servers have over 1 million ONLINE users actively receiving messages; this is mostly the official servers for major games, like Fortnite, Minecraft, and League of Legends.
In other words, while the MAU/DAU counts may be within the same order of magnitude, Discord's DAUs are more centralized into larger servers, and also tend to be members of more servers than an average Slack DAU. Its a far harder problem.
The chat rooms are oftentimes unusable, but most of these users only lurk. Nonetheless, think about that scale for a second; when a user sends a message, it is delivered (very quickly!) to a million people. That's insane. Then combine that with insanely good, low latency audio, and best-in-class stability; Discord is a very impressive product, possibly one of the most impressive, and does not get nearly enough credit for what they've accomplished.
For comparison; a "Team" in Microsoft Teams (roughly equivalent to a Discord Server or Slack Workspace) is still limited to 5,000 people.