If you followed the tutorial in this article, when you did makepkg --nobuild the imidiate output of the command should show the kernel verion for which it is downloading the source files.
Since your kernel version is 5.15.2-arch1-1 the the above command should show :-
~/build/linux515/master > makepkg --nobuild ✔
==> Making package: linux515 5.15.2-arch1-1 (Saturday 19 November 2021 12:10:56 PM)
==> Checking runtime dependencies...
==> Checking buildtime dependencies...
==> Retrieving sources...
-> Downloading linux-5.15.tar.xz...
Please make sure your kernel version and the version shown in the makepkg --nobuild command is the exact same.
(I edited the above command output to match your kernel version, your actual command output should look fairly similar to this.)
In case the kernel version in the makepkg output is different than your kernel version, then you have to start fresh, and BEFORE doing makepkg --nobuild you have to enter this command git reset iCommitNumber --hard where iCommitNumber is the EXACT git commit that has your EXACT kernel version. You said that you pulled the source files from here.
This branch has the source for kernel version 5.15.2-arch1 where your kernel version is 5.15.2-arch1-1. See ? Your kernel version has an extra -1 at the end, that means it is not the same as the one in github.
I think this is why you are facing the error. I think you should try to find the source files for your exact kernel or revert to 5.14 kernel and find its source files from github or aur.
Edit : I just checked the linux 5.16 kernel source files, and they have added the support for UB500 ! So if you cannot get the module to work, then just wait for the 5.16 kernel !