The guide that's publicly available says "If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor"
I think that's insane. Or, at the very least, surprising to encounter when working with someone else's code.
In general if you're going to work with other people I believe you need to be a bit flexible on coding with them rather than trying to be hard headed about your own style. It's important that people not be surprised when they work with your code.