It really is. Slack's interface is good enough and easy enough to understand that even the least technical people can pick it up in a few minutes after you show it to them and explain the difference between public and private channels. A week or two of use is enough to master everyday Slack use. Slack may not bring much to the table in terms of new functionality, but it does make it insanely easy for the average person to easily use the system and be involved, which really cannot be overstated when working at a company with a varying level of technical abilities.
But this is such a big deal. You make it sound like a good interface is trivial. It isn't, and it makes a huge difference.