That's a philosophy, not a fact. Individuals and organizations define things by all sorts of criteria. eg temporally, consensus and differentiation are just a few. It's not as simple as "we all agree this zebra is a human".
So is a pig that started with human genes, still human? Of course not. What about a human that started from pig genes? Hm.
That's what I mean, by being defined by existing humans. Because that's all we really have to go by.
You add wings to a person, lots of folks would be happy to call it a new kind of being. Some would argue its non-human. The proposal is to add disease resistance or other blood factors or cellular machinery, which are invisible and not obvious. But isn't it similar to adding wings?
"All humans that are possible" has to include the pig-human, and the winged-human, and so on. Clear to me, they are non-human.
Why are people so hung up on definition of human rather than more critical things we don't understand, such as consciousness and intelligence?
Perhaps they're afraid to be put in the non-human box, and non-human is to be removed or exploited? (Also, inhumane. For once properly built word.)
History has shown repeatedly narrow definitions of humanity in action. It ends in a bloodbath of some kind.