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).
To rephrase: the thought is that ecological thinking is nihilistic because somehow humans are "brought down to" the level of objects and that the value, purpose, and meaning of being human - that is, of being a subject and not an object - is negated. 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. In the end we are just objects in an ecological nexus (objects that happen to value things) and in the end that doesn't mean much when compared to the other objects in the nexus. Note: this is not my view.