Whilst it could be that the editor or author are technically wrong it’s also quite possible that they might be technically correct and were just arguing semantics here.
As far as geography/geology/hydrology goes rivers have a specific definition which the grand canal might not fit especially as one big system since afaik it extends and connects to multiple rivers and the water doesn’t flow in a single direction into a larger body of water.
I think the distinction the authors are making - correctly or not, I’m not sure, plenty of rivers don’t flow to the sea - is this artificial river flows into the sea, not into other canals or rivers.
The grand canal connects the Yellow and Yangtze rivers, for example.
The issue isn’t direction but rather having multiple branches as a river only includes channel(s) going downstream from a single point. Without that definition a river’s length would be the length of every river connected to it.
Thus the Mississippi and Ohio river are considered different rivers even though their waters mix and both flow downstream through the same channel.
According to this definition then Iraq’s man-made river (Saddam’s river) is longer at 565 km. the official role of the river was to collect irrigation water from Mesopotamia fertile land all the way to the Persian Gulf, though the real reason Saddam built it was to dry the Arab Marshes as it was difficult to contain the rebels in the marshes. https://ar.wikipedia.org/wiki/%D8%A7%D9%84%D9%85%D8%B5%D8%A8...
The longest one that I know of is the Grand Canal in China, which begain construction approx. 1400 years ago. https://en.wikipedia.org/wiki/Grand_Canal_(China)