We don't even know if he is dead or not.
Obito was pretty new to that mode when he attacked hiruzen, That attack could of landed before he implimented this "Destroy's all ninjutsu" thing into his attacks.
Since Tobirama regenerated from tobirama's first attack.
Wow, Tobirama attacked his self? This is some new shit.
mhm when he used that exploding talisman jutsu thingy he ended up getting blown up as well from it... But I think he only regenerated due to the fact that Obito didn't inflict that wound on Tobirama