It does not seem to do any patching to these files, only "install" into that folder.
Another place where you find headers is (example from my build folders)
openwrt/build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-ipq40xx_generic/linux-4.19.88/user_headers/include/
as stated in thhis part of the build system https://github.com/openwrt/openwrt/blob/master/include/kernel-defaults.mk#L107
These are the only two places you will find headers in, for each architecture/target you compile.
I don't know if they are patched or not, I guess you will have to find out by checksumming the files to see if they are different in the two folders
You can also try to ask in the mailing list or IRC chat as there is where actual OpenWrt developers mostly are.