Zajef on artifact sets?

Hi! so new Zajef prerelease video about varesa came out, and he says that if you are using codex, her combo needs to be different because she won't benefit completely from the buff, since not being in nightsoul blessing state. This brings codex to be around 1-5% worse and you need to use a different rotation..

I've watched this a couple times and still don't get how should I play her with codex, and I also remember seeing calcs on here that said codex was basically the same as her signature set..

could someone explain please?