It would potentially allow them to forgo the need to limit their UI to phone based with desktop upscaling. Make an app for the "best" small screen experience and the web site for the "best" large screen experience.
This strategy probably mitigates what might be a much more common complaint in the future that their "UI" sucks on mobile if they were to design for only larger screens.
This is the argument that I don't think is valid, though. When folks switch on "Desktop Site" in an Android browser, they expect that things will be squished, and that the site isn't optimized for the screen size. They just do it (or rather I just do it) because I want the full set of features, even if I need to do a bunch of zooming and scrolling around.
That is, Slack should still focus on making the website for the best large screen experience, but they shouldn't go out of their way to do somewhat sneaky stuff to detect my "Desktop" browser is really coming from my phone.
This strategy probably mitigates what might be a much more common complaint in the future that their "UI" sucks on mobile if they were to design for only larger screens.