Not the individual you replied to, but my company uses both internally. We used to use Skype (for softphones, video meetings, and IM), which sucked for group messaging. Slack unofficially picked up steam to fill the void, among some teams.
Now we've migrated from Skype to Teams internally. Groups that originally filled the void with Slack have pretty much continued due to momentum, but those that didn't have picked up Teams pretty enthusiastically. There's not really an advantage to one vs. the other, besides network effects and momentum.
We don't actually have an official Slack org, and but rather a smattering of Slack orgs (some paid, some not). But they're all used outside of the purview of IT, which probably isn't a good thing. Even the paid orgs are just expensed directly by various business teams, rather than centrally managed/invoiced by IT.
One thing of note is that we're an agency (well, a holding company with many agencies), and it's not uncommon for some of our clients to want us to sign into their Slack orgs and use it as a preferred means of communication. So our central IT makes the Slack client available for download, even though we don't have an official Slack org ourselves. This ease of access is probably one reason it started getting used in the first place.
Corporate uses Teams, Dev/Sys org use Slack. Mergers/Acquisitions complicate everything and ability to implement SSO across everything isn’t always speedy. Teams integration with Outlook is handy for some things, otherwise Slack is preferred - particularly due to preexisting integrations to pipelines etc.
We use teams for scheduled conference calls/meetings as it handles larger groups better than slack and outlook is setup to add team-meeting links into calendar events.
We use slack for team discussions and DM and sometimes for one-one calls.
I think this happened naturally and wasn't a planned path.
