I saw the 2015 keynote. I understood it to say it was being re-written. It's possible I could have misunderstood something
Yes, you misunderstood something there. Although I'm sure some parts have undergone major changes, no fundamental part of W10 has been thrown out and entirely rewritten from scratch.
Windows Vista was the only time in recent history where MS threw out and rewrote from scratch fundamental parts of the OS. That's one of the reasons it took seven years to release Vista.
You mean the 2014 keynote specifically stated that Windows 10 would only be UI/UX changes on the code side?
Yeah, MS really didn't say that. That really would have countered MS' marketing message, but I'm thinking the primary reason for not saying it is because it's BS. Here's a more accurate description:
MS has been on a path to OS unification ever since Vista. Up until W8.1 MS had achieved unification in two areas:
A) Kernel
Unification between phones, desktops and XB1 was achieved with W8/WP8.
The kernel is a chunk of code around 20MB in size on disk. It is a very important, but also a very small part of the OS. It resides at the very bottom of the OS' software stack and can basically be thought of as a driver for the CPU.
B) WinRT / WinPRT
~90% unification between phones and desktop achieved with W8.1/WP8.1.
Note that unification refers to parts of the WinRT library, CLR and the API surface. These things reside at the very top of the OS' software stack.
W10 is also about UI/UX changes, but it's even more about the software layers between A and B. For example, on the desktop, some parts of the WinRT library would call and delegate to Win32, but that doesn't exist on the phone. In these intermediate layers, W8.1 and WP8.1 are nothing alike!
W10 represents, for the first time, a unified OS where those components that are shared across form factors actually are identical. With W8.1/WP8.1 MS made it look to developers as if most of the modern OS stack is identical, but with W10 it actually will be. To achieve this WinRT no longer relies on Win32 but rather on something called Windows Core.
IMHO these are the most significant changes to Windows since Vista, in some ways since even before that. As is often the case for OS level changes though, the impact won't be felt by end users directly.