Agreed that the Constitution does not specifically define the term. The “elsewhere” the court mentions refers, IMO, to other sources of the time.
I don’t think 14A gives Congress the right to define citizenship, much less NBC. Enforcing a law is quite different from redefining its meaning.
The Civil Rights Act of 1866, 14 Stat. 27-30, enacted April 9, 1866, was the first United States federal law to define US citizenship and it affirmed that all citizens were equally protected by the law. It was mainly intended to protect the civil rights of African-Americans, in the wake of the American Civil War. This legislation was enacted by Congress in 1865 but vetoed by President Andrew Johnson. In April 1866 Congress again passed the bill. Although Johnson again vetoed it, a two-thirds majority in each house overrode the veto and the bill became law.