The question of why he would be called a "replacement" player is a matter of semantics. The term "replacement player" has no legal definition. The term "replacement" is loosely applied to any employee that crosses during a work stoppage and takes on certain legal protections when the work stoppage ends (e.g. the right to keep his job once union players return to work, etc.). So, to make a long answer even longer, the fact that a particular player has/had a contract does not preclude him from being called a "replacement player". The fact that he, irrespective of his contractual status, went to work at a shop during a work stoppage is enough to earn him the legal protections of being a "replacement employee". In your particular example, it makes him a "scab" in the vernacular as well.
And player contracts do not necessarily become null and void. During a work stoppage, performance of the contract is made impossible/impractical, creating the affect of being a free agent. As neither side is performing on their contract, the player is free to engage in activities that would not be permitted if the contract was active (as in, playing for another organization). Both sides can agree, if/when a new CBA is passed, to activate all the already existing contracts by integration into the agreement. So basically, they are not necessarily null and void so much as they are merely on pause.