IBM picks Slack over Microsoft Teams for its 350k employees

This article implies IBM's use of Slack has been some kind of secret? IBM was the catalyst for Slack Enterprise Grid existing.

IBM have been using Slack internally since about mid 2016 as a bunch of separate teams, set up by the individual departments who wanted to use it. At the same time, an internal CIO initiative called Whitewater started formalising tools to move the company away from internally-managed products such as Notes, Sametime and notes-database based reporting systems to "best-in-class" stuff like Verse/Outlook, Slack, Mural and Pagerduty.

I worked for the company until 2019, and was heavily involved in my department's initial use of Slack as a private instance, along with it's subsequent transition to the Enterprise Grid. There was never any talk of MS Teams while I was working there, and though I suppose it's possible they evaluated Teams over the last 12 months, there would have been far too much traction with Slack at that point to justify moving. IBM never does anything quickly. (It took literally 2 years to move every BU over to Slack and off Sametime, a tool that everyone hated, in the first place.)

I've wondered -- does slack harvest/share/sell employee data?

Is slack a tool that an employee can turn down?