“Historically specific ideas about who counts as an equal person”

This means that societies can have historically developed standards concerning who is regarded as a full member of the community and as entitled to equal standing.

Consider the United States before emancipation.

A Black person could be legally treated under a system in which:

Black people could be enslaved.

That legal institution itself reflected an extreme denial of equal status.

Then emancipation changes the legal order.

But the social assumptions that had developed around slavery do not necessarily disappear at the same moment.

The Library of Congress gives striking evidence of this: after emancipation, formerly enslaved people were legally free, yet they faced hostile whites, while later segregation and racial violence developed.

Rogers M. Smith gives you a useful conceptual vocabulary here. He distinguishes legal citizenship from lived citizenship and argues that equal citizenship involves not merely equal legal rights and duties but sufficient social, political, and economic standing for people to be recognized as civic equals.

So your argument is not:

“Society invents equality arbitrarily.”

It is:

A legal declaration of equality enters a society that already possesses social relationships, institutions, and ideas about status. Those pre-existing structures can affect whether the newly established legal equality is socially realized.

That is a much stronger and more defensible proposition.