The best part (and the part relevant to your comment) is that this just opens a Chrome window and you can open (cmd-N) as many windows into the same slack team as you want, and it behaves like you expect a Mac app to behave: cmd-tab goes between teams and cmd-backtick goes between windows of a single team.
I gather that there are some Mac apps or extensions or something that let you gather windows of _any_ app into tabs, so you could likely use one of those here to get tabs instead of windows.
Supposedly Chrome is getting rid of support for Chrome Apps, I think that's what these things are called (?). If so, I'm going to be impacted when it finally goes away.
Threading suddenly works in a non-stupid way, search suddenly works, and you can manage your own windows.