You're correct - it doesn't mean you're secure. It does, however, point out that you're putting some thought and effort into security. PCI requires remediation plans or justifications to pass, as does HIPPA.
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.
