Claiming that humans don’t have a “human nature” is only possible because there is a definable limit already present to delineate such a nature. That is to say, it is in human’s nature to make such claims as the presence or absence of human nature, and to insist that nothing/anything is impossible/possible, though the limits of both of those is of course (invisibly to most) proscribed and defined by those very boundaries of the human nature that all too many claim not to exist at all.
I recognize that leftist claims about the absence of human nature is in reaction to centuries of using the “nature” of various races to oppress and exploit them. However, to say that there is no human nature at all does not help any credible cause; it merely leads to doomed-to-fail utopian projects and the deaths of millions, much like the oppression they are nominally battling.
Human nature is amorphous, agreed, but is not all that changeable. Pretending it does not exist helps no one at all.