I’m pretty sure that the arbitrage possibility is made use of by building batteries to even out the duck curve.
You are absolutely correct, which is why these assumptions that building out lots of solar will result in free or even negatively priced energy make no sense.
Currently these arbitrage possibilities are infrequent enough that they can't be exploited profitably. There just aren't enough super sunny days where an absolute excess of power is generated that would make it possible for, say, a battery farm to get built out to capture that storage. But everyone knows storage will be essential as renewable energy makes up more of overall power generation, and at that point it will make sense for power producers to save up their excess "sunny day" energy to sell it at other times.