I agree the IIC+ gets sort of objectified as a metal machine, it totally can do all kinds of amazing tones, as evident by it showing up in Prince’s touring rigs, and Springsteen using one for like 40 years. While those guys created memorable songs, they didn’t really do much for tonal innovation, so the amp association generally flies under the radar.
Metallica is just a massively popular band that is widely known to have created innovative heavy sounds with the IIC+. It seems like an obvious association to make. IIC+ does killer heavy tones.
I doubt 2 in 10 millennial guitarists could even name a Sykes song, not to discount his significant musical contributions.