How is it Headcanon?
Cell outright says the Planet will be
no more. Cell tries to destroy the planet to “win” before this as Perfect Cell. In fact, Cell outright states “
he’s stronger than me, but it doesn’t mean I LOSE.” DIRECTLY COMMUNICATING to the audience that his Planet Destroying Maneuver (in his
vastly more powerful form) is NOT about overpowering Gohan. It’s about destroying the planet, and then in effect removing Gohan without having to be more powerful.
Even other characters who could have easily creamed Semi-Perfect Cell, like Vegeta and Trunks, could no nothing. Not because the blast would overpower them, but because the EARTH would be doomed.