OLE worked fine back in the 1990s because there was only really one way to make a GUI: Using a Win32 User32/GDI Device Context to render something while it received window messages representing mouse and keyboard input.
Now in Windows 10, User32/GDI is deprecated and obsolete for high-performance applications (and it’s really slow too after they axed GDI hardware acceleration in Vista), but there isn’t a single replacement, instead there’s like 5 of 6 - all incompatible with each other. While it’s true that it shouldn’t matter because OLE objects were rendered to their own separate hWnd, consider that the new UWP system doesn’t use hWnds, and frameworks like WPF have “air gap” issues with rendering their content overlaying another hWnd and often resort to hacks using window content blitting which drastically hurts performance (as you need to blit to/from RAM, instead of keeping it all in VRAM).
So if this approach is not allowing components to render themselves to a surface then it must be using an some other representation like HTML+CSS or something like WPF’s XAML such as Silverlight - and that has its own set of problems (e.g. any HTML+CSS will be outdated in 5 years, and we all saw what happened to Silverlight.
If this is web-only, then that means this is the beginning of the end for the rich desktop experience for Office - but it’s also irrelevant because we’ve had <iframe> for over 20 years - and the new Web Component system is gaining popularity.
I understand the business and technical reasons for doing this - but Microsoft never replaced GDI with an _approachable_ replacement. Direct2D is nice, but it's very difficult to use from the .NET world, whereas GDI is a first-class library feature in `System.Drawing`.
I get nested Office documents all the time, sometimes taken to extremes that even Microsoft might not even had thought about, inception style.