Is this really a big issue? Im in the US and the offer is from a silicon valley company thats doing ok.

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.

It's not a huge issue, but it's worth minimizing your exposure/risk, so to speak. In other words, I would just keep your options open and continue to entertain other offers. 6 months is a very long time in Silicon Valley. All kinds of things can happen (the person who offered you the job gets fired, there is a restructuring, the company undergoes a merger/acquisition, it downsizes, it pivots in a different direction and no longer needs you, or it scales up and ends up needing to fill the role faster with somebody else, etc etc).

By their nature, tech companies know what they need today, predicting what they need in 6 months doesn't always go well.

I'd consider it an issue. Most companies that are hiring are hiring for immediate needs. Did they explain why they want you but don't want you now? Make sure you understand their reasons so you can assess the risk. Are they counting on funding coming in or an increase in revenue? Funding doesn't always come and business plans change.