No, archaeologists discovered that it was originally built as a keyhole shaped mound with a moat and a length of 250 meters. It was partly destroyed and reshaped as a circular mound when the Imperial Palace of Nara (Heijo Palace) was constructed in the 8th century.
Ichiniwa Kofun is designated as the tomb of Emperor Heizei in the 9th century. Of course, archaeologically, it is correct that it was built in the Kofun Period, perhaps in the 4th century.
By Noboru Ogata, Kyoto University.