By that definition, if Steig used McCulloch's calculation, then a failure to acknowledge McCulloch would seem to me to be a probably minor form of plagiarism. (deepclimate.org)
But although federal officials have said «ghostwriting» may be a form of plagiarism, which is prohibited, it's not clear that the Office of Research Integrity (ORI) would act on this particular case. (sciencemag.org)