Why is he calling Doumeki naive?

I think he's implying that they will obviously fight for the guy both groups are looking for, there's no point in making deals or asking what to do next.

ok, that's what i thought but him saying his name threw me off.

I think saying his name was meant to expose his lie about not remembering D. Y pretended not to remember D 4 years before. This is him acknowledging the lie that they both knew was a lie.