That is a LOT of methane.
Indeed it is. Methane is everywhere in the universe in great abundance. It is everywhere we look in our galaxy and in our solar system. Most of our neighboring planets and many of their moons have large supplies. It is a constituent of comets. It is inconceivable to me that Earth somehow escaped having methane incorporated into its formation along with water - an even more abundant molecule. Earth's methane is not from stewed dinosaurs and ferns, it is a fundamental part of it's original formation.