Also, I think it makes sense for Microsoft (or another company) to use their own products (such as their Teams service as a replacement to Slack). Best way for the company to know what their customers experience.
Do you know why? That seems to be a weird thing to refuse to admit.
The reason given in the article is solid, though. "Don't share confidential info on systems we don't have control over/trust in, especially if they are controlled by a direct competitor".