First of all, what save function are you talking about?
Second of all, the point is that the whole concept of 'new' when talking about this sort of software doesn't really hold up very well. It's something that's constantly tweaked and iterated on, never throw away and replaced. Or at least not the engine as a whole.
The ability to save your game of course. Save bloat is a constant problem in Bethesda games, it was especially bad in Fallout 3. I've lost so many characters because the save file was corrupted all of sudden and they didn't fix it in Skyrim either, hopefully it'll be fixed in Fallout 4.
I know it doesn't hold up well, Bethesda was just talking out of their arse, if they'd had said an upgraded or tweaked version of their current engine then I wouldn't be complaining. To call it a 'new engine' however, complete and utter horseshite.