What does “historically meaningful argument” mean?

It is a claim about a real historical process that can be investigated and supported with historical evidence, rather than merely being an abstract philosophical statement.

For example:

“Freedom is a philosophical concept.”

That's abstract.

But:

“The abolition of slavery did not immediately cause white Southerners to treat formerly enslaved African Americans as social equals.”

is a historical claim.

You can investigate it through:

contemporary testimony;

laws;

court cases;

newspapers;

political debates;

racial violence;

segregation;

historical scholarship.

So “historically meaningful” means historically substantive and empirically examinable.