running the top 10k passwords on each hash will likely get quite a few hits, and not take much compute time... the overlap to accounts that are owner/admin accounts is unknown.. just the same entirely possible. (not counting for slack's password complexity requirements)
Going by the paper linked in Xylakant's comment (https://news.ycombinator.com/item?id=9277780), if the cost factor was 12, using dedicated hardware would let you test 52 passwords per second. So that would take 192 seconds to try 10k passwords against a single hash. If you were to run this for a month straight you'd have tested 13675 accounts. Slack has over half a million daily active users (and I'm not sure if that stat actually means daily active accounts, or daily active people (who may have multiple accounts on different teams)).
The paper goes into estimated costs as well, but I'm not going to dig through it to figure out how much it would actually cost to run that hardware for a month straight. And of course it's talking about dedicated hardware, which Slack's hacker almost certainly doesn't have.
