Yep, Windows XP icons were designed and produced on a Mac in FreeHand.
Microsoft was our first paying client back in 1997: we made the icon suite for Outlook Express on Mac OS 9. It was the email counterpart to Internet Explorer 5.
https://news.microsoft.com/source/1998/01/06/microsoft-announces-march-availability-of-office-98-macintosh-edition/
https://techhub.social/@maccatalan/114155754052920748