I don't assume it. I know it for a fact; I've met some of their team and I know others by reputation. And I'm not exactly a slouch when it comes to this stuff (I don't eat and sleep crypto but a large part of my business is building secure infrastructure/consulting on the systems running on that infrastructure for regulated as well as non-regulated environments).
Which is more secure?
A) Slack.
B) Open source software on a LAN accessible only through physical entry, SSH, and/or a VPN.
Then I suggest you put more effort into securing your LAN situation because that is a vote indicating your belief your workstations are insecure.
I was also a bit surprised what they consider out of scope for their bug bounty program: https://hackerone.com/slack
They've sent bug reports with credit card data they've typed in during a phone call through a variety of insecure methods.
They've also written people's credit card info on sticky notes.
Trust me, the horror that is card data and a call center is scary.
Compliancy only tells a story about management and how many MBA's you have, it doesn't actually mean you have good security. Only being compliant isn't going to help you not get data leaks or data loss!
And, for better or worse, you need your service providers, including chat, to be compliant. If your company were to leak PII via Slack, your company would be in pretty hot water for putting PII on a non-certified service provider.
At least if it were certified, you could say "we've done our due diligence to protect people's PII". Perhaps only important to leadership and lawyers, but still important.
There's no question that Slack's budget is greater than my own. They have a large, full-time security team. I have a bit of attention from myself or a colleague when setting a system up.
There's also no question their expertise is better. These are life long security professionals with direct experience at other SaaS companies.
All we can do is assume that Slack cares about security enough to be sufficient. Last I checked, they didn't have any form of compliance certification, yet HIPPA, PCI, etc. compliant clients use them without reservation.