Or HaChaim on Deuteronomy 34:7/אור החיים על דברים ל״ד:ז׳

1 בן מאה ועשרים שנה במותו. he was 120 years old at his death. This means that at the precise moment his soul left him Moses had completed 120 years. Another nuance the Torah may have in mind by writing the word "at his death" is that seeing the Torah reported only afterwards that neither Moses' eyesight nor his vigor had diminished that this statement was true up until the moment Moses died.