“American historical scholarship celebrates Lincoln because American academic institutions favor narratives compatible with liberal nationalism.”

This is a specific sociology-of-knowledge hypothesis.

It could mean that institutions systematically reward interpretations presenting Lincoln as a national liberator and therefore give less attention to interpretations emphasizing other consequences or interests.

But to establish that proposition, you would need evidence about the institutions themselves.

For example:

university curricula,

textbooks,

hiring patterns,

funding,

publication patterns,

scholarly debates,

treatment of competing interpretations.