Originally Posted by Sai'ix Renkai
Well, it could be one of many things. Here's two that I think.
To release from the Reaper, you need to sacrifice your body, not your soul. So he did just that.
Or, he sacrificed what remained of the soul of the body he took currently, right before the Zetsu. I mean, do we know that he COMPLETELY takes over? Maybe there's some soul leftover. Who knows, aside from Kishimoto?
Using the mask seems to be the preferred way to summon da reaper.