Pros and Cons of Slack aside, what are they spending their money on? Say you start of with Java+Jetty code, PostgreSQL, and Redis as a proof-of-concept then tune it into a production app. Chat rooms are constrained in size well defined in Slack so horizontal scaling shouldn't be any big deal. Have somebody cobble together some open source C++ libraries for video (I assume that's what they're doing) and deploy that as another app. You end up with sales, chat server development, client development people, the C++ video server, UX designers, and maybe 100 mid-range servers. I don't see this totaling $256.11 million a year.
The really dangerous thing right now is long-term unemployed software developers. Some of them might decide to re-architect Slack, Teams, Zoom, Facebook, etc. with an expected annual operating cost of $40 million a year.