Does an overbite always 100% mean that a person sucked their thumb

1] no
2] most human beings have some form of overbite [ it is worse in some people ]
3] moderate to severe ones are corrected with orthodontia
3] ask your siblings, close friends - put teeth together, and see how much space there is between front teeth and bottom teeth [ usually 1 centimeter or so ]

Nope

no
i have one (sorttaa) and i didnt