[Barrin92]

Many people enjoy candy too, doesn't mean it's good for you. Not everyone is a software engineer, but everyone has limited attention.

Slack's chat nature as the OP points out favours instant messaging over batching up replies, which, like many bad habits, appeals to the reward portion of our brain but is genuinely unhelpful in structuring work. There's a reasonable (and increasing amount) of evidence that multitasking and context switching can lower your working IQ by 10 to 15 points. Deep Work by Cal Newport does a good job of going into the detrimental affects that distraction from workflows has on people.

[warp_factor]

you put it better than I did in my initial post. They managed to hack that reward part of the brain with immediate interaction at the expense of deep meaningful work that require long periods of reflection before producing anything.