There is indeed a 20 segment limit on OW RTW tickets ex-Aus (as elsewhere in the world).
My guess is that either they are wrong, or they have another RTW non-OW fare which includes a lot of the OW airlines (eg BA/CX/QF).
If they do have a OW RTW ticket with 28 segment limit I am sure many of the FTers on this board will be smiling with glee to get their hands on that one......