Most likely, this comment was added in response to a botched attempt to simplify code, to serve as a warning for future maintainers to think twice before making a similar attempt.

The commit that added the warning was "Add note about space-shuttle code style"[1], and the one before that was "Revert controller/volume: simplify sync logic in syncUnboundClaim"[2]

[1] https://github.com/kubernetes/kubernetes/commit/de4d193d45f6...

[2] https://github.com/kubernetes/kubernetes/commit/8a1baa4d64ca...