That's exactly where it cme from.
In the published accounts of Kerry's contempraneous journal, he began Dec 24 at one of the Mekong mouths (In the South China Sea!), proceeded up river, on one engine to conserve fuel, reaching Sa Dec (55mniles = as the flying dog flies - from the border) around midday. After refueling, "wasted" half a day at Sa Dec, moving out in the early evening. This doesn't give enought time, even at flank speed, to reach Cambodia and return to Sa Dec. (The return, in time to close the jounal entry - still on the 24th - in noted in Kerry's account written at the time.