Domain Functional Level and AAD Hybrid Join

Good Afternoon!

I apologize if this question has been asked before, but I have searched for the answer and just want to get some clarification. 

This article states, "The minimum required domain controller version for Windows 10 hybrid Azure AD join is Windows Server 2008 R2."

https://docs.microsoft.com/en-us/azure/active-directory/devices/hybrid-azuread-join-plan

Does this mean the minimum domain functional level needs to be at or above 2008 R2, or just that we have to have servers at that level? I assume this means functional level, but wanted to confirm that.

We are in the process of upgrading our functional level to 2016, but that will be about 3-5 months out. We want to roll out Conditional Access before that and are hoping this won't hold us up.

I apologize if this seems like a low level question. I have set up Hybrid Join in the past, but this is the first time I have run into an environment with this specific scenario. 

Thanks!

Sean