This isn't the source-code of QE, this is just a tribute.
You can see the "main" repository, with the rest of CP/Mish, where this was taken from at https://github.com/StalePixels/cpmish/tree/ESXDOS (the working branch this code was developed in) - which is a fork of David's original code at https://github.com/davidgiven/cpmish. This repository is just a mirror of required files to build the editor without the rest of the Operating System. The licence documents for the entire of the original repository are here too, to make it easier for you, the reader, to find.
Bugfixes against this QE itself should be made to the parent repository, bugfixes for the NextZXOS shim version of libcuss (the ESXCUSS folder) are welcome here.
Building is handled via the Makefile - not part of the usual C/PMish way of doing things, and this project was last successfully built with z88dk 2.3 (v22110-51889e5300-20231220) by @mquin.
If you want to help contribute to a bigger, more powerful editor, maybe consider https://github.com/StalePixels/EviL/ which is a fork of QE, with plans to add more Vi Adjacent Features, and Spectrum Next specific hardware enhancements.