alt Hacker NewsI think what you're missing is that they could all be focused in one spot, not spread over the city. If curved reflective buildings can melt siding, and mirror solar plants can melt salt, I'm pretty sure a city's worth of sun focused on, say, a college campus, could start a massive fire
I see your concern. The mirrors could be deliberately manufactured to have enough imperfections, or slight reverse curvature, to prevent this. It's possible with properly-designed optics to prevent the mirror from being able to focus on a spot smaller than, say, 1/50000 the mirror's area on the Earth's surface. Make each single mirror only optically capable of focusing down to a 3-mile diameter at best. Then it would be optically impossible to focus all the mirrors to a single 60ft spot, and the theoretical highest brightness at any point on Earth would just be regular daytime levels of light.