What is most likely is the price of a 10GPON ONTs and the OLT optics is such that installing it now is roughly equivalent to putting in the current ONTs and 2.5G OLT optics, so it makes sense to put in the newer gear as then you won't need to touch it for literally years if ever again!
I would also guess that Chorus get a fairly good deal from Nokia on the gear since then Nokia can say "We've successfully deployed 10GPON in NZ and it has been a resounding success" as part of an international case study to talk with larger GPON installations in other parts of world. As we are only talking about new deployments vs the existing one which for a roll-out that is mostly part is complete. Chorus (and the other LFCs to a lesser degree) have shown that they can successfully handle a nationwide GPON deployment so them being able to deploy 10GPON with the associated 100GB uplinks. Also means that Nokia sell more 100GB backhaul gear too which considering the price of SFPs sounds pretty good.
While I could be having an incredibly pessimistic view on it all, but I am not sure.