Why won't the job start for another 6 months? Get a good answer to that.
And things can move fast in Silicon Valley. Maybe they now think they honestly need you in 6 months, but in 6 months, that could change. And they still have 6 months to find someone better or cheaper.
The same goes the other way aroundd of course. A job offer now is worth a lot more than one in 6 months. You could find a better job and not be available in 6 months. If they really need you, I'd expect them to hire you sooner. Maybe in one or two months. Some companies operate slower than others, but 6 months? That's a lot.
If I were you, I'd try to fill those 6 months with another job or with freelance work. That gets you money and experience. If the other offer is still there in 6 months, you can still quit the temporary job.