GP is basing their comment on 5 year old data. As of April 2025 the duck curve problem is solved - https://blog.gridstatus.io/caiso-solar-storage-spring-2025/. Mid day demand increased because of battery charging, smoothing out the load. The California grid is looking immensely healthy, being a net exporter most days.

Will it handle an extended heatwave that also affects other states simultaneously? You’re right, we can’t know that with certainty until after it happens. But based on what I’ve read I’m confident it will.