You forget that mythbusters did their test with limited budget and construction time, based off a historical rumor.
Now, a fortress with years and unlimited materials, creating a MASSIVE shined funnel over a 20x20 area that focuses it all into a shaft down through a final lens into the forge, that will have more power to it.
Despite worse technology, the dwarves can compensate with a massive scale. So what if it is impossible to make a mobile parabolic mirror ignite a ship?
Make a nonmobile construction of hundreds of mirrors that focuses it through a lens, designed to be most effective at ~15 minutes of the day(though it can work the temperature up throughout the day as it approaches the optimal point) COULD WORK.
Plus, you are assuming that the sun in DF is equal strength to the one in real life, whereas in DF it is an unknown factor.
Yes, solar powered forges would be impractical and limited, and require a massive megaproject, but...
- impractical
- limited
- multi-year
- massive
- likely failure
That same checklist
applies to DEFINES every single MEGAPROJECT, but that doesn't stop players from making them, even just as a show of power and an unstoppable will to outdo common sense.