Doesn't macOS round the corners of application windows, including browsers?
Interestingly, Edge doesn't apply rounded corners to its own content presentation, either, as can be seen in the screenshots provided in a page (Neowin) linked by another comment, here.
To me, it appears much so as if MS is making a serious attempt to differentiate between own and foreign content, which includes a rather surprising comeback of visually prominent frame/window borders, which are marked by round corners in this particular design language. (If this is a good thing or not is left to the beholder, but it's at least interesting.)