The reason for this is probably because the UI team uses Mac. If you look at the cruiser icon on this Fluent 2 introduction page, it's clearly a macOS one with vertical lines on the gloves.

Pretty sure a lot of the web Office, Outlook, and Teams stuff uses this design. It would make sense too since Teams is built with React.

Teams' UI is schizophrenic garbage. It's the Calvinball of UX. There's no consistency or rules. There's only laughably awful performance and text boxes that don't automatically take focus when the application is brought forward.

The only thing that can improve Teams is for it to be deleted and all backups of the source code thrown into an acid bath.

This is what strikes me the most about all Microsoft promotional material. For a long time now they have been showcasing beautiful applications and interfaces in promotional videos and demos, but then the final products end up looking or feeling nothing like them, always feeling simplified and cheapened.