This kind of response to security researchers just invites the next researcher to sell the exploit instead, or to actively exploit it.
Why does Slack seem like a company that is floundering? It took them over two years to release a simple feature like shared channels. It seems like the app is frozen in time and the company is doing nothing except keeping the lights on and waiting for Teams to obliterate them.
Slack turned from a hungry tiger startup into an exhausted lumbering enterprise giant whose primary weapon is litigation and mudslinging (Slack initially encouraged the Teams competition, then filed suit against Microsoft in perhaps the biggest case of corporate sour grapes in some time).
Pay your security researchers properly, Slack.