This statement is missing context. Under One China policy (the status quo between the CPC and the KMT up until 2016), the Taiwan Strait was considered territorial waters with the median line used to delineate the divide. In the recent years leading up to 2016, neither party crossed this line.
In 2016, the DPP took power and declared that One China policy no longer holds, thus, that the Taiwan Strait is an international waterway.