There is nothing to suggest they can't compare the two.
The giants aren’t Oda talking. The giants are the giants talking.
Just like when Kaido was talking it was just him talking… when Kaido says Roger was top 1, he’s talking from his perspective… He’s the one placing Roger above Shanks… But again, HE DOESNT KNOW ABOUT IMU… So… Kaido can put Roger above shanks but he can’t scale Imu who he doesn’t know shit about