I'm not sure what this is, a mistake in the program, in that it accepts this
or simply a redundancy in the docs? In the docs in sec 6.5.8  Light Groups
one of the examples is:

#declare RedLight = light_source {
  <-500,500,-500>
  rgb <1,0,0>
}

light_group {
  light_source {RedLight}
  sphere {0,1 pigment {rgb 1}}
  global_lights off
}

The whole lightsource syntax has already been declared so saying
light_source{RedLight} is not necessary you only need to put:

light_group {
  RedLight
  sphere {0,1 pigment {rgb 1}}
  global_lights off
}

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