It's entirely and easily conceivable that there could be another species that looks at us and wonders whether we are intelligent or persons according to their standards of intelligence and personhood. After all, manipulating our environment might not be proof enough; it could be argued that we still do it as an automatic response, simply working together as a species to propagate the species and still ultimately motivated in all our actions by innate, animalistic desires. It can easily be argued that the only fundamental difference between humans and other animals is simply the complexity of our interactions with our environment. Even the things we hold up as proof of transcendence from the animal world are likely just adaptations, eg, what we consider an impulse to develop ethical systems is just an adaptation of the brain to keep individuals working in the interest of the species.
Furthermore, imagine these hypothetical creatures had a better understanding of what we consider "souls," death and even pain. They could theoretically see no ethical issue with causing humans pain, suffering and death since, from their perspective, we don't really understand what's happening and our reactions are just automatic self-preservation.
Intelligence only lets us know whether or not we can do things, not whether we should do things when it comes to matters of morality. Intelligence tells us how to capture the dolphin and keep it captive not whether what we are doing is right. Intelligence tells us how to exploit other people not whether it is right.
I think you are spot on that accepting other life on this planet is a prerequisite to accepting life on other planets (I think this is illustrated beautifully in District 9, BTW), but intelligence has got nothing to do with it. It takes a great amount of intelligence to mobilize a genocide effort but you would need to ignore some of our universally accepted morals.
For a person or species that possesses great intelligence but little wisdom, empathy, and other attributes that enables us to do what is "right" self-preservation and exploitation is likely the default.
But I don't think the scale is smooth. There are sharp divisions: the use of tools, the understanding of death, the use of language, the use of language to describe abstract concepts, the use of oral history, the use of written history, the creation of artificial intelligence aides, etc.
I absolutely agree that having a bigger picture than just "people are special" is a prerequisite for first contact, I just also think that going to the other extreme, that all intelligences are mostly the same, is probably much worse than just having a human-bias. Instead we'll need a graduated system with different privileges and rights depending on where species appear on the scale. Such a graduated system will be the basis to determine whether or not we're being treated fairly by E.T., if/when they ever show up.
If true, that leads me to think that we'll need some sort of version 2.0 of some of the major religions that deal with varying levels of sentience. This will probably happen after contact and not before, so it should be a very traumatic event for everybody concerned.