I mean... sorta. It's not easy to write a Office add-in just using the C API, and when I tried a year or two ago the only example code I could find was like 20

I mean... sorta. It's not easy to write a Office add-in just using the C API, and when I tried a year or two ago the only example code I could find was like 20 years old and all documentation was heavily pushing people toward the newer JS-based add-ins. I quit that job because the extensibility story for Excel was so painful (but they had good business reasons for being in the space), sigh.