I think it's less about the "ecosystem" they the bought into but rather the bean counters saying "Hey, we have free access to Teams, why are we wasting our money on Slack?" (That might have been what you were saying but it read more like "We love all things MS, why not move our chat to that too?")

No doubt several factors/motivations were/are at play, but I think what Parker is talking about is that for companies/orgs (particularly big corporates and government bodies) who already had a Microsoft operating environment and who weren't already using Slack, they would be much more comfortable adopting Teams, for reasons of cost, interoperability and (perceived) security, than adopting Slack, even if Slack was superior as a standalone group chat app.

That’s fair, I was more-so talking to the companies that have dropped Slack and moved to Teams (I have a number of friends who have experienced this first-hand).

At one point a company I used to work for tried to get us to move to Hangouts (or whatever Google was calling it that week) since we already paid for Google Workspace accounts for everyone. Thankfully there was enough pushback to kill that idea early.

Your counter to "ecocsystem" is literally "because ecosystem"

Wanting to switch into an ecosystem because of cost is different from wanting to switch because it integrates better with other products. For myself, when I hear "ecosystem", I think more of the latter which is why I commented. Cost might be a factor but it's not the first thing that comes to mind for me when using that word.