Huh, that's interesting. I've just started using Outlook again for the first time in 10 years, but haven't seen any Excel previews so far. I'm guessing there's an option somewhere to turn that off. And it probably depends on Excel being installed.
(But having lived through the days when previewing a mail in Outlook would cause any attached scripts to be executed without user confirmation, I'm hesitant to preview anything in a piece of software coming from Redmond.)