It says "citizen" for Congress because a naturalized citizen is eligible. That's the only distinction to draw from those words.
I agree.
He doesn't believe there are multiple classes of citizenship. He believes there is only one class, citizen and that all citizens enjoy the same rights and protections under the Constitution. Where he distinguishes a difference between citizens is in the multiple paths to obtaining citizenship.
For example:
1. Citizens by birth
a. natural born citizens
b. citizens by the 14th Amendment
c. citizens by statute
2. Citizens by naturalization
3. Other citizens by statute
An example of this might be derivative citizens - i.e. foreign woman who married a U.S. male between 1855-1922. Or a woman whose husband became naturalized in those same years and the wife became a citizen by statute automatically - this one could also be seen as a sub-category of naturalization, but there are probably more examples.
The Fine Print: I believe I have acturately summarized his opinion. However, don't hold him accountable for any errors in my interpretation.