[throwaway7312]

Cooper actually says "a people built this", not necessarily insisting it was humans or descendants of humans (though that was my impression too the first couple watch throughs).

The other thing to keep in mind is he does not actually have any way to know who (or what) built the tesseract.

Just like how Brand earlier states the spacetime curvature hand she shakes hands with is "the first handshake" (implying first contact with an extraterrestrial), but it actually just turns out to be Cooper. The characters are confident of things they think are right, without actually being totally reliable narrators.

[cgriswald]

Well, that's... incorrect. Cooper actually says, "They're not 'beings.' They're us," and later says, in response to TARS saying that people couldn't build the tesseract, "No, not yet, but one day. Not you and me, but people." I really don't see any wiggle room in the actual events for claiming Cooper believes they are anything but human; or at least beyond human. But he sees them as an extension of humanity.

Whether Cooper is right may be an open question, but the film heavily implies it. That 'first contact' turning out to be Cooper may be evidence of wrongness, but we don't necessarily need that since we know the characters are not omniscient, and in the context of the film it implies that there aren't any aliens. There are also no signs of any other life, despite several habitable planets. Frankly, the film doesn't even make sense if "aliens did it"; any message of hopefulness about human spirit would fall apart.