The title is a bit editorialized. Microsoft is doing the equivalent of embedding a "Web View" in Outlook. Instead of having some stale custom build of MSIE, they're using Edge because it's already installed and they're in control of the API and its compatibility. They're also offering a setting to disable this behavior.
The links don't "open in Edge". That would suggest they launch the Edge app (instead of the default browser) and open the link in that. Instead the links open in a pane in Outlook that embeds Edge (presumably with the same settings and session context as the actual app). This also only affects the desktop Outlook app, not the far more modern and less clunky web version. I genuinely wonder how many HN users commenting on this story actually use desktop Outlook app or know someone who does and doesn't also use Edge (or their IT department's mandated out-of-date copy of Firefox ESR).
Now, bear in mind I'm saying this from a position that is in favor of splitting up Microsoft (and Google and maybe Apple). The feature is certainly useful if viewed in isolation, but it is in effect anti-competitive behavior because even if they wanted, they couldn't provide generic integration of your browser of choice the same way and the new behavior is opt-out rather than opt-in. It's bad, but let's not pretend it's worse and more deceitful than it truly is, just because you already don't like Microsoft (and presumably don't use their products).
This is probably a genuine usability improvement. It's also anti-competitive. Both of these things can be true at the same time.