This study is aimed to re-examine the Interface Hypothesis via investigating the adult L2 acquisition of the word order variation of numeral classifier indefinites at the syntax-semantics and syntax-discourse interfaces in L2 Chinese. A computerized acceptability judgment task was administered to 41 advanced and intermediate adult Korean learners of Chinese to collect data in accuracy and reaction times. For accuracy, it was found that the advanced L2 learners have not acquired the target phenomenon to a native-like level and did not score significantly higher than the intermediate L2 learners at both the syntax-semantics and syntax-discourse interfaces. For reaction times, while no significant differences were reported between the syntax-semantics and syntax-discourse interface items for the advanced L2 learners, developmental progress was observed on the syntax-semantics but not the syntax-discourse interface phenomenon. The findings suggest that the internal versus external interface dichotomy should not suffice to account for the acquisition difficulty and processing costs of different linguistic properties in L2 acquisition.