Or a more concrete example of today, many people today define themself by being anti green, anti progressive, anti woke, antifashist etc. but often struggle to define what they are standing for.
The theory would be that given A believes X and B believes Y, A is more likely to incorporate ~X and B is more likely to incorporate ~Y.
And because beliefs tend to be on a bit of a continuum, what starts with A believes Z <= 4 and B believes Z >=5 evolves to A believes Z <=1 and B believes Z >=9, and both believe anyone who believes 3<=Z<=7 is not part of their group.
group A believes in X
group B believes in Y
What the parent poster seemed to imply was
group A believes in not Y.
Or a more concrete example of today, many people today define themself by being anti green, anti progressive, anti woke, antifashist etc. but often struggle to define what they are standing for.