I guess it's not suitable for every team though.This is the key probably. I can understand it doesn't work the way the author used it but personally I'm fairly happy with it (not to say it's the best, it's just one of the chat apps which mostly just works) and don't use all bells and whistles.
We use slack for 2 main things, 1 is chatty stuff which doesn't go to the issue tracker which has the important stuff which still needs to be there in a year (seems the OP had like everything in Slack, probably a source of problems mentioned) and due to me being a freelancer at that company it's perfectly normal I don't answer in a day or more. We learned to use it like that so no FOMO going on. The other use is as a 'dev/ci status channel' which is only viewed at passively: it gets all build info, PR/issue notifications etc which is a great overview, for us at least.