I thought this was true (it makes some sense), but the data seems to show the opposite. This source has cast iron at 52 W/m-K, and 1% carbon steel at 43 W/m-K (0.5% carbon steel is at 54 W/m-K). Now, this obviously depends a lot on the processing and heat treatment of each material. But at the very least, the difference is not massive.

https://www.engineeringtoolbox.com/thermal-conductivity-meta...

I guess most of the reputation of cast iron comes from the chunky shapes of the pans, and that the higher end stainless cookware is often cladding around copper core