I’m not familiar with the way nix does locking, but it is possible that this file is created with the permissions set so that even root cannot delete it. This would be a safety feature rather than a security feature: root can change the permissions to allow himself to delete the file if he really wants to, this just makes it a little harder so you think twice before doing it 
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Can you ls -alFh /nix/var/nix/profiles/default.lock?