Not sure how the network drive part would fit into that hypothesis, though.
This is all just speculation. I don't code for Windows, and I don't know anything more about this vulnerability than what's stated in the advisory.
The vulnerability, as I understand it (I did a little research by examining one of the vulnerable applications), since we don't have any actual PoCs to examine:
notepad.exe, like many of the stock Windows apps, uses a bunch of system libraries.
One of those system libraries loads shdocvw.dll (an Internet Explorer related component).
shdocvw.dll has a delay-load dependency on a library called 'ieshims.dll'. On my computer, when I start notepad.exe, shdocvw.dll tries to load ieshims.dll, and fails, but continues normally.
This means that since ieshims.dll is not found in the app directory or the windows system directory, the search for it will continue all the way into the current working directory, which would make it possible for an exploit to put a payload in an 'ieshims.dll' stored in the current working directory (next to the .txt file), and it would then be loaded.
Assuming my research is correct, this looks like a security vulnerability introduced by one of the Internet Explorer developers, probably for some sort of compatibility purpose. They ignored the fact that IE is used as a component in many system applications, and basically added a vulnerability to every app using shdocvw (there are a lot of them).