I think he's being a little bit to hard on Slack. He was really stretching a free trial offer beyond its limit. If he was a paying customer then I might have cut him some slack.
What I understand is that author did a trial and hit technical limitations in the desktop and mobile applications, I think that paying extra would not had help. Probably the slack developers did not tested with such a large number of users, it reminds me when I worked on a rss feed application and we were testing with 40-50 popular feeds, then a client shows up with more then 1000 rss feeds , I end up grabbing his feeds and testing with all of those and optimize the application(We did not expect someone would had so many feeds).