I'm not sure those two mean real downside protection - but your point is correct - the VCs are betting on the very small chance that Slack is actually adopted by a huge number of businesses. Right now I'd guess it has a fairly narrow adoption among leading edge companies and SF/Valley natives. I'm sure there are exceptions to this - or maybe they truly have moved beyond that core audience and that's what is driving the valuation.
But that's what VCs do - make a large number of bets that will fail and one that blows it out the park.
Their valuation is a reflection of the growth in Slack's business - meaning they have strong enough growth that they can command that kind of valuation on a huge raise. It is also like a reflection on the limited number of companies that have that kind of growth.
But these kind of valuations also create a high-wire act for the companies in my experience. Growth must be maintained at all costs to justify the valuation. They spend like they're going out of style because of this.
