absolutely, I would be fairly certain it was stored outdoors and got wet more than once before you received it.

DF is the least of your problems if;
The post were not galv. or powder-coated and the concrete was not properly coned to prevent water accumulation.

I believe the rest of the fence was constructed well. The metal posts were painted and of high quality, I've seen other works by the guy and they were fine, including concrete.

So if it will be stored outdoors throughout the mild southern California winter, will it still be onto stain and seal in like March? It's November now. It just tends to rain a lot this time of year and it's cold-ish.

Don't worry about it. Do the staining in spring when the weather is better and the wood has had a chance to dry out a little from being rained on. Really, it will be fine. It would be fine if you waited a couple of years, in fact.