With something this hard it's extremely tough to tell for sure when it will happen and often progress goes in chunks where it seems like it's going to happen but then doesnt.
Even CEOs who are an experts are by their very nature too biased to take at face value.
citation required
Although one should never believe the ravings, useful work does surprisingly often nucleate around them, just not to the degree or with the speed promised.
If the expert works in a lab developing new experimental solar panels they probably don’t see a clear path to mass production.
The CEO might know another manufacturing expert that does see a path to production and have enough high level understanding to know the methods are compatible.
Like a CEO saying, I have a material that can protect wearers from nuclear fusion blasts and it will be on the market in 6 months, but the experts in the field have yet to actually prove that material exists and create it in a lab.
But the point is that we are all equally capable of being wrong. Especially when we step outside our area of expertise. CEOs are just another type of expert but their domain is organization. We have to consider the source’s experience relative to the domain in question before we can decide if their prediction is trustworthy.
If I wanted to defend Musk I would do it explicitly. You don’t need to read between the lines.