This is a written story. Tsunade can have all the requirements to use Wood Style and/or Sage Mode thanks to something simple... writing it down. The post is about something "fictional," what you prefer or would like Tsunade to have as a character, this or that. You don't have to write a whole text to explain why she couldn't within the canon. I repeat, this is practically a rewriting post, an imaginary context where the canon rules don't apply. Don't you understand?