In this case you could do it either way, either set the tub first or install the pony wall first.
When we install a tub, we put a 2x4 flat; if that is the right height of the concrete pour, and then pour up to that. I don't like setting a heavy tub on concrete if it's that thin. The 2x4 makes a nice resting place for the apron.
Then you can finish tile up to the tub.