I think there is a thing called Microsoft Office development kit or some other name that allows C# and other .Net programs to manipulate Office apps. Not sure if it's what the author meant, though.
Oh, I have used OLE (or some iteration thereof) to drive Excel from Python. That was a long time ago, but it sounds similar. I wasn't really doing GUI, though - I was using code to build spreadsheets from data files.
I had something I wrote in the 90s I wasn't proud of that was a word macro that read Excel sheets for a list of instructions and used those to compose documents and print them. I feel sick thinking about it.
