It's Slack + low-latency voice chat. With vastly less hassle than Teamspeak/Mumble/etc. Recipe for success right there, though I'm not sure how well their monetization has gone.

I haven't heard this term before, but I'm not sure it is a "rob and duplicate" if they are targeting a market segment that Slack never advertised towards nor was really designed for. Seems more like refreshing Slack for a niche, but it turns out to be a really big niche.