Except the Victoria service in the morning peak appears to continue to Blackburn, not Leeds. So unlikely to see 170s at Southport in the morning peak.
Okay then, whether its a 4 car 150 or a 769, it still isn't 5*23m, so as I said the platform lengthening requirement may have changed.
I'm pretty sure the lengthened platforms weren't due to be in place for the December 17 recast (postponed to May 18) so I don't think you can draw accurate conclusions from looking at a subject to change timetable for May 18 on OTT, especially considering most of the 170s will still be with Scotrail and it's rumoured the few that will have arrived will be used on another service until 195s enter service. We also don't know if the Alderley Edge to Wigan/Southport service will continue long term or whether it's an interim measure.
There are still 2 x evening peak services to Southport from Victoria which will originate from Leeds.