I agree with the idea of the article. But it will not be limited to slack, check out the home page of this Ride Sharing service for example: https://www.wadeeny.com (disclosure, I am a co-founder)

I am the one who designed it, and I can confidently say that engagement is way higher than using traditional forms and button. The bounce rate is extremely low, almost everyone at least tries interacting with the bot.

Now this is not a testimony that text will always beat traditional web UI, I think a big factor in this case is the fact that it seems so unconventional and people are curious to try it out. Also depends on the purpose of the interface you are creating. Like 1971genocide pointed out, UI is good for giving commands and not learning for example.

In our case, it was important for us to get a limited number of very precise information from the user. Therefore having a Text based interface worked out.