The term is open to interpretation, but it certainly doesn’t simply refer to non-christians.
Some interpretations view the antichrist as a specific individual or figure who opposes Christ in some hypothetical, end of days type situation, while others see it as a broader symbolic representation of a certain figure or person that represents the complete opposite of Christ’s teachings or the spirit of Christianity.
“Anti” can mean “opposite” just as much as it can mean “against”.