What he actually is saying is that since we don't have the source code, creating it from scratch would takes many months. Decrypting or building from scratch would be a tedious thing because you're spending time decrypting the game to recompile it, or you're using the source code to recompile it, and since source codes aren't released from the game developers, the only option is to decrypt and recompile, which isn't as easy as it sounds.
From what i understand, it's possible to decrypt 3ds/NDS games easily, but recompiling it to a 3ds format is a lot tougher and i don't think many (or if anyone) has successfully done it.