They did in fact get seniority numbers when they signed the contract with CX that also entitled them to expat terms. CX a while later said sorry, no longer the deal, local terms and no seniority until you join as a Cadet SO. They had already been instructing in ADL for some time. Yes it is an odd situation but there was a route to join CX this way, which they did. CX then changed the terms after the fact.
I got my seniority number in ADL, whilst flying FTA aircraft, one month before stepping foot in HKG.