Multiple mirrors reflecting sunlight can indeed result in an irradiance greater than that of the Sun (this is how solar power towers work). But to do that, you need to cover more than a 0.5° circle of the sky with mirrors. The total irradiance can be as high as you want (up to covering the whole sky with mirrors, which would be equivalent to standing on the surface of the Sun) but the irradiance per angular area can never be higher than the Sun’s.
You can think of it as dividing a mirror of any shape into a bunch of flat mirrors. Each mirror can only show you a reflection of part of the Sun (which is exactly as bright as that part of the Sun) but you can have as many mirrors as you want showing the same part of the Sun, multiplying the light you receive from it (up to filling the entire sky once again).
Multiple mirrors reflecting sunlight can indeed result in an irradiance greater than that of the Sun (this is how solar power towers work). But to do that, you need to cover more than a 0.5° circle of the sky with mirrors. The total irradiance can be as high as you want (up to covering the whole sky with mirrors, which would be equivalent to standing on the surface of the Sun) but the irradiance per angular area can never be higher than the Sun’s.
You can think of it as dividing a mirror of any shape into a bunch of flat mirrors. Each mirror can only show you a reflection of part of the Sun (which is exactly as bright as that part of the Sun) but you can have as many mirrors as you want showing the same part of the Sun, multiplying the light you receive from it (up to filling the entire sky once again).