The ambiguity that I found in ATPL requirement was in the wording of SASP Part 2 para 6(a)
500 hours as pilot-in-command under supervision, or 250 hours as pilot-in-command PIC of aeroplanes of which up to a maximum of 180 hours may be as co-pilot acting as pilot-in-command under supervision (P1 U/S).
With an MPL, you can only act as co pilot so as per this wording only 180 hours can count.
If my interpretation is right, then out of 250 hours PIC, if max 180 may be as P1 U/S then that leaves the need for 70 hours as PIC in airplanes and out of 500 hours, it leaves you with a shortfall of 320 hours as you only have co pilot privileges on an MPL.
With a traditional CPL, CASA (Aus) has an eligibility requirement of 70 hours PIC and CAAS approved CPL course needs 100 hours PIC so that covers the CAAS ATPL requirement when you fly for the airlines as one could easily make the remaining 150/180 as co-pilot acting as pilot-in-command under supervision (P1 U/S) and get an ATPL. Also with a CPL, you have PIC privileges on a commercial level, so you could be able to log the 500 hours as pilot-in-command under supervision.
STAA have a total of 80 hours total flight time in their MPL program.
I don't know if my reasoning is correct then all they need is to make up that deficit of the remaining PIC hours in an airplane under private privileges that an MPL offers. I guess its not that that simple and I might be missing some pieces of the puzzle as I'm sure if that was the only reason TigerAir would have rectified it unless they have no requirement at present for Capt. who would need an ATPL.
So either SIA would have to hope CAAS amends the rules or they would have to see to it that their cadets get atleast 70 hours PIC if not more in an airplane in their training program.