This doesn't mesh with how I feel toward Slack. Slack itself is the thing that's disrupting my productivity and deterring deeper thinking by pinging me all the time. When I open Slack, the vast majority of the time, I'm looking to express my point as clearly AND concisely as possible so I can get back to doing whatever it was, which is 99% of time more important. As a result I use (or don't use) grammatical conventions, capitalization, and emoji depending on the context of my message and what will allow me to get my entire message across the fastest, and hopefully correctly.
If I'm messaging the person I've pairing with all day, or if I'm coordinating with my team about who will pick up a story, I'm much less likely to give my message all the proper formatting. These people what I'm like and how I tend to communicate, and so I can rely on that prior knowledge to "cut corners" here and there. I'm also much, much more likely to use emoji because I can communicate my intentions and my "intonation" nearly instantly, without having to sit there and worry about wordsmithing my whole message.
That being said, if I'm message in a large public channel that perhaps has multiple teams or belongs to a team I don't normally interact with, I'll spend much more time wordsmithing and editing to make sure I'm following proper conventions - mostly for the sake of clarity.
It does make some very salient points (think about how others could misinterpret your message, expand acronyms/use them sparingly, write to your audience, etc.) but I think they bought a little bit too much into the "Slack has replaced email" argument. For me, the main benefit of Slack over email is the lowered barrier to entry to starting a conversation and the ability to keeping it flowing, and part of achieving that is being okay with lowering the standards of communication.