LED Brightness or Luminance is additive or not?
You're comparing apples and oranges when you refer to 'brightness' and 'luminance'.
'Brightness' is 'how bright is the light when I look at it', while 'luminance' is 'how will lit is the surface the light is shining on'.   

Luminance is additive - if you shine more lights on the same surface, the surface gets more light.  

Brightness is not additive - putting a light next to another light doesn't make either of them brighter.