Not really. Slack would have to have a real, native app that used exclusively stock OS controls. I very much doubt they'd do that, so there would still be a lot of work required to implement dark mode.
Most Cocoa apps use system controls.
It's honestly not much dev work at all (took me a couple days on a medium size app). The real time sink is with design, because virtually any custom color you're using in the application will need a dark variant.