Is there a reason why I couldn't just reference a CDN like this in my production code?
No, there is no reason not to use CDN in production. This is even the preferred way to serve content in production mode, especially with common packages like jQuery, since most people have already downloaded and therefore cached this resource.
The employee suggested that it could only be for development, and that unpkg just translates "on the fly" (which is not very good for performance).
- CDN!:) , , , . , , , , .
, , :)