Wouldn't the simplest decision be that Brady is telling the truth? That he hasn't completed decided yet.
I think there are 3 possibilities:
1.) Brady is going to retire, but was not planning to announce it to the world the day before the Conference Championship games. He would probably want his agent to work out something with the Bucs regarding his signing bonus and timing of filing of the retirement papers before making anything public. It's easy for us to say that Brady/Giselle can afford to leave $15M on the table (which is almost certainly correct), but he may feel very differently than we do about this point.
2.) He truly hasn't decided. Which in its own way would be notable, because in past years he would conclude the season by basically stating he's gearing up for the next one.
3.) He wants to play for another team (e.g., 49'ers), and his agent is working behind the scenes to game out the possibilities.
I'm in the camp that it's 80/10/10 with regards to the percentages for the above scenarios, and that 80% is my low estimate.