Copy of production

We are currently using CRM Online 2016. We want to create an environment that is exactly equal to production so that we can test Update 1.

I was thinking of using steps below but got feedback that "you can copy a production environment but the copied environment will be in a managed state and you can’t develop on it." That sounded quite a strange limitation. Is it true?

https://technet.microsoft.com/en-us/library/dn659832.aspx