To say that humans are amongst beings and not correlates of beings, for some, means to say that humans are downgraded to the status of "objects." The center-point of experience, namely: the subject, is thus treated merely as an object among other objects, merely one being among other beings. If humans are nothing more than objects, in the eyes of some, this means that the human loses its unique place as something special in the universe - a special place inhabited by that being which can cherish meaning and value, pursue with intelligence goals and purposes, and create a life: all acts that enable the human being to be referred to as a distinct and special creature apart from others in the universe. (Presumably other things, like rocks and pens, but also animate organic forms of life like flowers or puppies, do not inhabit such a central place in the universe because they cannot cherish meaning and value or pursue with great sophistication and intelligence goals and purposes like humans are able to do, i.e. through the ability to "reason.")
To rephrase: the thought is that ecological thinking is nihilistic because somehow humans are "brought down to" the level of objects if other beings are granted similar subjectivity once only afforded to humans. Thus, what is special about being human is lost.
Perhaps humans may value experiences, create meanings and so forth, but that just doesn't amount to much in the grand ecological scheme of things if other creatures, in their objectivity, are just as much subjects as are human beings. In the end we are objects as much as we are subjects - but also subjects as much as we are objects: a basic point brought home by the German idealists and the existentialists.
In the end however the question posed (the more interesting question) is: does the fact that human beings in their cherishing of meaning and pursuit of goals make human beings somehow *more* valuable or unique than other meaning-valuing creatures? Than other objects endowed with a subjective life? Ecologically speaking, of course the answer should be "no."