Effective altruism is based on the core belief that all people count equally. We unequivocally condemn Nick Bostrom’s recklessly flawed and reprehensible words. We reject this unacceptable racist language, and the callous discussion of ideas that can and have harmed Black people. It is fundamentally inconsistent with our mission of building an inclusive and welcoming community.
— The Centre for Effective Altruism
This feels to me like it is begging the question, so I am not sure I understand this principle. This framing leaves open the whole question of "what determines how much capacity for things mattering to them someone has?". Clearly we agree that different animals have different capacities here. Even if a fish managed so somehow communicate "the only thing I want is fish food", I am going to spend much less money on fulfilling that desire of theirs than I am going to spend on fulfilling an equivalent desire from another human.
Given that you didn't explain that difference, I don't currently understand how to apply this principle that you are talking about practically, since its definition seems to have a hole exactly the shape of the question you purported it would answer.