His argument is theoretically correct, but fails in real-world applications.

theoretically 是由名词 theory（理论）加上形容词后缀 -etic 和副词后缀 -ally 构成的副词。theory 来自希腊语 theoria（观察、思考），-etic 表示"与...相关的"，-ally 表示"以...方式"。theoretically 意思是"在理论上、从理论上说"。