lastly I just tried your version of .ssh/config and the exact error message I got was:
git clone ssh://aur@aur.archlinux.org/foobar.git Cloning into 'foobar'... The authenticity of host 'aur.archlinux.org (5.9.250.164)' can't be established. ECDSA key fingerprint is SHA256:L71Q91yHwmHPYYkJMDgj0xmUuw16qFOhJbBr1mzsiOI. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'aur.archlinux.org' (ECDSA) to the list of known hosts. Permission denied (publickey). fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.
which clearly states you have an issue with your publickey which is an ssh-issue...
Could also mean he hasn't uploaded the public key properly into his aur4 account.