No, a simple password like "slack123" should be easy to crack with any usable password storage method.
Not necessarily easy. If they're using a decently high cost for their use of bcrypt, we're talking hours to days (or more) per user, even when only considering weak passwords like that.
True, I guess it's possible to crack a password for a single user, especially one with a weak password. I was more thinking that it's unlikely they'll be able to crack the passwords of everyone who was in their database, and given that Slack has so many users it's unlikely for any single person that his/her password will be cracked.
Of course, even if they can't steal everyone's passwords, maybe the hackers will try to crack the passwords of higher profile targets.
GPUs are fast enough to crack a very large percentage of passwords in a short time by brute force, if a simple algorithm was used, even with salt.
With a separate salt for each password not even the NSA can crack that (that we know of). With a single salt for all of them, maybe.
Of course, I barely know anything about computer security, but at least it should prevent attacks using rainbow tables I think?