Quite right.
The Bellei case had nothing to do with expatriation from the U.S., whether voluntary or not. Had Bellei completed his U.S. naturalization he would have expatriated from his native Italy.
That's correct, but the armchair lawyers conflate denaturalization and expatriation, or flat out are unaware of the difference, and argue that the Bellie case does not involve the fact of naturalization - that the case is only about loss of citizenship, not getting citizenship in the first place.
Bellei was denaturalized, not expatriated (from the US). The legal standard for those two changes in legal status are radically different, and covered in completely divergent lines of cases.
A natural born citizen cannot be denaturalized. Bellei was denaturalized, therefore Bellei could not have been a natrual born citizen.
That's a roundabout way to get to the point that Bellei was naturalized in the eyes of the court, but it does add the dimenstion of noticing the law surrounding his loss of citizenship, which is what the case was argued over. The case was "can Bellei be denaturalized?" not "can Bellei be expatriated?"