You don't know if Tobirama made that technique, and also, that technique is a suicide one. Also, White Snake Regeneration; Oral Rebirth.
Or, or, or, or, he was studying it by using the life of others, and chakra.Do remember that Orochimaru based his jutsu of Resurrection on theories and formulas previously formulated, possibly by Hiruzen or Tobirama.
Given Hiruzen was shocked Orochimaru was studying resurrection jutsu's when he encountered him doing those things.
He's only missing senjutsu. How Hashirama has it but Tobirama doesn't is hilarious.
He probably can use senjutsu but can't use Sage Mode that well.
Considering the fact he knew how to counter Obito with senjutsu and recognizing Mt. Myoboku toads.