Hey Bob,
as far as I know, they didn’t say not recommended, but not supported.
Here in Germany we use this for 3-phase high-frequency spindles: LAPP ÖLFLEX® CLASSIC FD 810 CY. It has double shield and it is specially designed for use in Power Chains. I use 4 × 1,5 mm² (3+PE) for 2.2 kW.