How is this absurd? I've had problems debugging minified Javascript in production too. So from my point of view, this seems like an appropriate response.
In addition to the speed difference, there's a significant payload size difference: something like 1.5+MB for the dev build, vs 70kb for the production build, last time I checked.
And about the size: if the JS file is cached properly, downloading 1.5mb once doesn't seem like a huge issue, especially for something like Slack where only desktops use the web client (most mobiles probably download the Slack app). I don't know if this was the case here though.