I think this article completely misses the value of Slack. It's IRC made easy. Very easy, and that makes it very valuable. It's like the iPod, which did everything MP3 players already did, but without being a pain in the ass.
All work exactly the same way, and all have the same feature set.
Push notifications let conversations happen in real-time. You can turn off Jira, Github, Trello notifications and set up integrations in Slack, create #firehose channel, and now you have corralled all those announcements into one spot.
If only I could get the other members of my team to use it, I could maybe talk about its chat features.
You're coming at the "value add" from a very technical perspective. Which is fine, but it doesn't add weight to the argument. Do you think that a significant percentage of Slacks user base even really know it's based on IRC? Not too many of them are firing up irssi, pining for a better web based IRC client. Indeed irccloud has been around longer than slack and has nowhere near the adoption.
The product is solid. The brand is great. Nobody* gives a shit about the protocol sitting behind the glossy surface.
*Nobody meaning a significant percentage of paying customers.