Nothing is a bottom type, which means it extends every type, but doesn’t allow you to create an instance of that type (because that would be a nonsense class).
The definitions of Some and None might help to clear this up:
final case class Some[+A](x: A) extends Option[A] {...}
case object None extends Option[Nothing] {...}
As None extends Option[Nothing] it can be used in place of an Option regardless of the type.