Security issues are said to be the thing that makes Microsoft give the internal memos to their employees. They can not use the products owned by competitors.
As stated from the notes in the document, the apps that owned by the competitors, such as Slack does not give control functions that can protect Microsoft‘s intellectual wealth.
Moreover, that famous software maker also does not suggest using the Amazon Web Service and Google Docs products.
Thus, Microsoft also prohibits the use of GitHub for the shipment of the very secret info or code.
The use of Grammarly is completely prohibited too. For your info, Grammarly is a software for checking grammar and spelling to analyze the neatness of the typed text.