Depends on how it's used, and DOOM's Megatextures are different and far better. I know DOOM 3 used Megatextures but was still moddable. All it comes down to is that it might be harder (custom maps can be a possibility because DOOM 3 allowed for that without megatextures). Plus, this might be something they have planned for future games and have yet to finish.
As for Rebellion working on this, people do realize Codemasters responded too, right? It's not uncommon for developers to respond like this. So it really means nothing. Honestly, I'd be more surprised if the independent developer (which I still highly doubt as there's way too much going against this idea) actually was allowed to respond. You know how Bethesda can be sometimes.
I quickly jumped on the Rebellion train, but the more I think about it, the more I think it is being developed internally by Id. Bethesda Softworks has done extremely little work with outside developers since the spending spree of Id, Arkane, Tango, and MachineGames. Certainly nothing on the AAA level. Additionally, making Rage 2 instead of a DOOM sequel would be extremely low on the list of fiscally bizarre decisions made by Bethesda Softworks.
All I know is that we need more of that Id Tech 6 goodness. DOOM and Wolfenstein TNC are easily the smoothest-running games on my PC.