It's less about whether they can technically handle the scale (they clearly can), but more about enterprises that are going to have typical enterprise demands - security policies, encryption at rest, whitelabeling, maybe even on-premise installs. On top of that, enterprises typically aren't self service, you need to have a pretty large sales team to effectively sell to the enterprise.
obviously, but part of that investment is increasing sales velocity to enterprises. It would behoove slack to grow and get more bigCos entrenched onto the platform - switching costs and employee preference can help them get a foothold on the market to beat back competition.
