I believe it is because Canyon suspension strut is so long and the design may put the shock shaft under load, which would lead to excessive wear. Maybe Fox and Canyon are overly cautious. You would not really want to differ at the shock stroke (and overall length) for sure.
BTW, I have an aluminium frame bike and my shock info says "Description: 2020, FLOAT DPS, P-S, A, 3pos, Evol LV, Canyon, M64 - Spectral On, 230, 60, LCL, LRL, CML, Stealth Logo" So, in my shock all damping valving is "low". I think in this case I could very well run a coil, maybe even with a normal spring. The original ones had more progressive rear suspension kinematics. Sadly, I have not found the actual graphs. Canyon themselves say that 2020 ones had much more linear geometry.
It would be nice to learn what kind of valving the carbon frame shocks have.