That seems unsafe. It assumes that the editor is started via a shell command, and not executed directly. But there is no actual binary /usr/bin/: but there is a /usr/bin/true; I would use that instead.
It assumes git works the way that git works, and that git will not break backwards compatibility in the future. I think the same can be said of any alias in git.
The question is, is this behavior documented to run the command in a subshell? If it’s only documented to split on spaces and expand environment variables, the git command could change the implementation at any time.