It's not; the authors statement seems to use what I udnerstand to be the standard definition used in the field, where a gene is sepcifically a sequence which codes for the synthesis of a particular protein or nucleic acid sequence, which is essentially the lowest level “simple” trait. This is perfectly consistent with large and overlapping sets of genes being involved in determining each of the complex traits that humans mostly care about, and does not make the term “gene” meaningless.