I think he means “perfectly tuned to the way fans want it” which is to say “highly moddable.” Skyrim is kind of the first game in the series that sold really well on platforms other than the PC which strangely brought in a lot of fans who play the vanilla game. But as far as I can remember, the bulk of the longterm fanbase plays on PC and installs tons of mods for the game.

Sure, there are other games that fans like to mod (Minecraft being a big one) but I can’t think of any other game where fans stack dozens or even hundreds of mods by different authors all on the same game and actually expect it to work. The fact that it does work at all (and fans have created custom programs to merge mods and to carefully tune the loading order) is rather a miracle!

So this is what I think he means by “perfectly tuned.” A brand new engine would mean putting in a ton of work to support all the different forms of modding fans want to do and in all likelihood would be far less flexible and powerful, leading to modder community outcry.