I'm still betting that you need an ATPL. I doubt that Jet2 would make such a mistake on their pilot hiring webpage. The fact that the apprentices just went through not long ago would back that up.
Why would they call a 1500hr pilot (ATPL requirement) an 'ab-initio' yet a 500hr pilot (TR/non-TR FO) a DEC/Non-DEC FO? It's a mistake. It's meant to say fATPL aka CPL/IR. I wouldn't read too much into that. The 'Second Officer' bit "would back that up."