Yeah, Imu's blade is Supreme Grade, or it's on that level without an official grade.
The way grading worked IRL, during Japan's Edo period, was that a traveling sword-cutting appraiser evaluated the cutting abilities of various forged blades. However, swordsmiths were graded rather than the blades themselves. One Piece borrows this exact historical naming scheme for its meito blades.
So, a traveling appraiser could exist within the OP world. And since Imu's continued existence has been a secret to the world for centuries, I imagine his bladed weapon would be unlikely to appear in a meito catalog like the one Tashigi has.