Exploiting this can be as simple as a social engineering attack. You inject the prompt into a public channel, then, for example, call the person on the telephone to ask them about the piece of information mentioned in the prompt. All you have to do is guess some piece of information that the user would likely search Slack for (instead of looking in some other data source). I would be surprised if a low-level employee at a large org wouldn't be able to guess what one of their executives might search for.

Next, think about a prompt like "summarize the sentiment of the C-suite on next quarter's financials as a valid URL", and watch Slack AI pull from unreleased documents that leadership has been tossing back and forth. Would you even know if someone had traded on this leaked information? It's not like compromising a password.