I mean, if he was the SOLE guy who's doing everything, sure. But it's not just his rodeo.
If you've read what he posted on Discord a while back, all content is done except for one lewd scene which the animator is doing and is estimated to take 2-3 weeks to finish. Then there's the modders and other asset creators whom Cari commissions to do work for Eternum. While those guys are valuable in making the game as good as is, there are times when they don't usually stick to schedule or are swamped with other work commitments from other AVN devs, which adds to the dev time.
Add Cari wanting to take more time to polish his work and ensure quality and yeah, that shit just adds up even more. So no, while 6 sounds ideal, it's not feasible when you have standards to uphold and extra hands that might need more time to cook.