I'm not sure what the situation is now, but for a long time Outlook used MS Word, not MSIE, to render HTML email messages. Word is a terrible web browser, so that made it next to impossible to design templates for things like email newsletters that would look & behave sensibly in Outlook.
For people that aren’t in the know: the MSO engine is basically a very incomplete and quite buggy implementation of the HTML 3.2 specification. From where I stand, it looks like the entire thing is a binary blob that essentially hasn’t been touched at all for over twenty years. Literally.
Microsoft switching back to MSO breathed new life into an entire industry of people with esoteric, hard-won knowledge about the eldritch affairs of HTML for email. Without it, HTML for email would not be such a disaster. (Sure, there are other awful email clients, but Outlook is by far the most terrible that is used much these days. If it had tightened its game, other bad clients would have been much more like to fix theirs too.)
Outlook renders HTML for email but doesn't use IE.
This, of course, was a very smart decision even though Outlook's rendering of HTML is terrible.
That's probably because unlike most Microsoft products, Office is cross-platform and has to support multiple operating systems, including MacOS, iOS, and Android.
The Office team did a couple of session at CppCon about how they refactored the code.
"CppCon 2014: Zaika Antoun "Microsoft w/ C++ to Deliver Office Across Different Platforms"
https://techcommunity.microsoft.com/t5/Office-365-Blog/Share...
The latest office for Mac looks quite a bit like the windows version now (for once) so it’s not too surprising.
Office does a lot of things their own way and without a lot of the cross-compatibility that's a feature of most Microsoft products.
That's why you'll find you'll be able to do something in Office, but other Microsoft programs won't. Or vice versa.
Like Notepad and Internet Explorer are pretty much just containers for certain Windows controls. Controls Office don't use. They roll their own. Or at least, used to. Wouldn't surprise me to find out that Office had their own browser rendering engine that didn't rely on anything in IE.