The worst thing about it is how fucking easy it would be to fix. Just treat the backticks as characters when backspacing or navigating with the arrows, that way you can 'enter' or 'exit' a code block reliably, and all previous muscle memory from the markdown editor would carry over.
But no, apparently that's too hard for a company that's worth more dollars than the number of seconds any of us have been alive.
I think there's a battle between nerds and non-nerds, and maybe in between is the worst place to land. I know some of my projects' users complain a lot about the lack of a WYSIWYG editor in our forums and issue tracker (we use Markdown, displayed plain until previewed or saved). Some people hate it. But I would hate a standard WYSIWYG editor, so we just accept the hate. Every Markdown-ish WYSIWYG editor I've tried has sucked...so I don't know that it actually is easy to get right (if it is even possible, where is the good example?).
Slack serves a mix of nerds and non-nerds, with non-nerds becoming a bigger and bigger portion of their user base over time. I can only assume it will become less and less enjoyable for nerds in service to the goal of serving their growing non-nerd users. For my own projects, I don't foresee myself ever using Slack (I use it for work). It feels like a decent product getting worse with time as it tries to be all things to all people.
But no, apparently that's too hard for a company that's worth more dollars than the number of seconds any of us have been alive.