This is probably not the repository that you are looking for.  This repo comprises a discontinued experiment to change the design of QuantLibXL.

If you want to compile QuantLibXL from source code, then the repo that you need is:

https://github.com/eehlers/QuantLibAddin-Old

And the documentation is:

https://www.quantlib.org/quantlibaddin/build_qlxl.html

That doc explains how to retrieve the source code from packaged tarballs.  You can instead retrieve the source code from the QuantLibAddin-Old repo and the build is otherwise the same.