The short answer
Stewart Butterfield sent the memo to the Tiny Speck team on 31 July 2013, two weeks before Slack's preview release, and published it later. Its argument is that Slack was not competing in a defined market with clear incumbents, it was defining one, so tweaking the product was not enough, they had to tweak the market too. The line people quote is that what they were selling was organizational transformation and the software just happened to be the part they could build and ship. Practically that meant the pitch led with a reduction in the cost of communication and zero effort knowledge management, not with features. If you are selling something buyers do not yet have a category for, this memo is the clearest statement of what you have to do instead of a feature comparison.