I agree that there is more too it. But, it is difficult to pin down. The factors that go into this kind of solidarity are a mix of intrinsic ones and those that we attribute. The comparison doesn't make perfect sense, but neither does any definition of human, or a rationale for putting non-humans on similar footing.I've never felt comfortable with this comparison. My mother works with mentally handicapped people and she once said something similar, essentially comparing the mentally challenged with the cognitive capacities of a chimpanzee. But this comparison does not make sense. A chimpanzee that is particularly intelligent doesn't become "more human" and a cognitively challenged human doesn't suddenly become "chimpanzee-like". We are completely different species with different genetic make-up, and with humans also inhabiting a symbolic order that is pretty much completely absent in the world of the chimpanzee.
IMO, I think it is precisely this that makes us human, this flexible/dynamic symbolism that allows us to have this conversation - to construct conceptual frameworks that can be debated and critiqued over time. Chimpanzees will only ever have a solidarity of mind with their kind, but humans can symbolise totally new solidarities, i.e. all apes deserve fundamental rights, all mammals, all organisms, etc. That is totally an evolving open-ended symbolic process, i.e. what is "similarity of mind" can always be re-articulated, depending on where we are intersubjectively as a species.The comparison doesn't make perfect sense, but neither does any definition of human, or a rationale for putting non-humans on similar footing.