Actually, it's both. If, say, the USAirways contracts are voided, the flight attendants for that airline would be striking when the contract had been voided.
Your point is also well taken, the flight attendants for all other airlines would be striking while under a valid contract. This would be the equivalent of what the PATCO Air Traffic Controllers did under my Freeper namesake, President Reagan. Striking under that set of facts is a well-established replacement worker situation.
Reagan80
PATCO were federal employees statutorily prohibited from striking; it didn't involve a contractual situation. PATCO strikers weren't "replaced" - they were discharged.