That's not what it implies at all. It was Jarul who told them he has the Nidhogg fruit. They wouldn't have known otherwise.
Imu definitely recognizes the fruit from having seen it before. He's like double Jarul's age. This means that despite Loki being an ancient Giant pirate for 8 years never revealed his power to someone who could report it to even the Gorosei, who would have reported it to Imu like Luffy's power was reported to Imu.

