After `su user`, getting `fatal: failed to stat : Permission denied` with `git push`

There's a lot of **setup** for this question: I've got a host (rpi5.local) with 2 user accounts: pi and cake. I wanted to explore git, and I created the cake account to "own" the "server/origin" repos. As user cake, in /home/cake, I created a folder (git-srv) with a sub-folder called projectA. I went through the process of initializing this repo using git init --bare. Initially, I "populated" the projectA repo on the server from another host (rpi4b.local). I pushed some files in a folder called Aproject on the raspberrypi4b.local host to the projectA repo via SSH using this sequence:

$ hostname
rpi4b.local
$ pwd
/home/pi/Aproject
$ git push -u ssh://git@rpi5.local/home/cake/git-srv/projectA.git

This worked fine. As user pi, I cloned the projectA repo to a folder in /home/pi/XYZ. I made & commited some changes to one of the files in /home/pi/XYZ, and attempted to push those changes to the server as follows:

$ hostname
rpi5.local
$ pwd
/home/pi/XYZ
$ git push -u /home/cake/git-srv/projectA.git

I got an error from this effort:

...
error: remote unpack failed: unable to create temporary object directory
To /home/cake/git-srv/projectA.git
 ! [remote rejected] master -> master (unpacker error)
error: failed to push some refs to '/home/cake/projectA.git'

After a bit of research on this error, I concluded it was a *permissions* issue between pi and cake. I *figured* the solution would be to su cake as user pi, and so I did that, and tried the push again. And finally, my question:

$ whoami
cake
$ hostname
rpi5.local
$ pwd
/home/pi/XYZ
$ git push -u /home/cake/git-srv/projectA.git
fatal: failed to stat '/home/pi/motd.git': Permission denied

What does failed to stat mean in this situation, and is there a straightforward *work-around*?