Is it likewise unfair to use the Bible as an indicator of what Christians think? No, of course it's reasonable to address codifications and official documents as evidence of what the relevant groups think.

Secular humanism isn't as stringent a religion as Christianity, but the manifestos were written with the intention of summarizing what that kind of person is likely to believe. Presumably, the leaders of that organization had some idea what they were talking about, and were familiar with many secular humanists.

This kind of nominalism can be pushed too far. In your case, does it have a basis in libertarianism?

I don't remember the exact point I was making about the superficiality of humanistic egalitarianism, but I doubt that point was based just on quotations from the manifestos. The latter would have assumed that cooperation is possible. My point would have been based on criticisms of the humanist's aspirations, which means my reasoning would have been independent of this issue of whether the manifestos are important. So it's not a question of whether I "know" the basis for that egalitarianism is weak or superficial. The question would be whether my arguments to that effect are solid, or whether secular humanists have strong arguments for egalitarianism.