Even when R=G=B in a well behaved working space is neutral, that in no way means sending those values to a printer will produce a neutral on output.
In an RGB working space, if all three values are the same, its neutral. But you don’t send an RGB working space directly to a printer, it needs to be converted for that device. HOW its converted, what the resulting RGB values become (not R=G=B) is the key here, its what good ICC profiles are supposed to provide.