It’s because the higher you are in the chain of corporate command, the less time you have to dedicate to each task. You end up with shorter answers to every note because you wouldn’t have time to reply to all notes and do the strategic things you need to do, otherwise.
As an individual contributor on a team, you may have to interface at most with 30 people on a weekly basis. As a second line leader you may have 150 people under your purview, and another 50 outsiders you have to talk to. You can’t scale the amount of time you have, so you scale the amount of time you spend on replies.
Shorter answers don't necessitate terrible grammar. Maybe it's because my mom was a teacher and I had good grammar drilled into me, but I feel like it shows respect for the people you're communicating with.
That doesn't explain the "punctuating with multiple cryface emojis".
Using the example from the article: "K let circle back nxt week bout it . thnks"
I'm not buying your argument. The amount of additional time that it would have taken to write that same message with proper grammar and spelling is minuscule.