I would state that I think the belief of this (in those times) was accurate.
Researching and having just presented on NP reefer traffic, this thought was held by the early engineers of my studied railroad who for a little while after PFE was using other insulation materials, were still using the separation of >dead air spaces< by stapling felt, rosin paper, tar paper, to wooden frames and using thin wood panels to create these prior stated, dead air spaces. And wood is having pores, would have this dead air space.
And all of that above construction would be somewhat more difficult to do in a steel car than a wooden car. At least to people who are used to dealing with wood, and skill level is that of working with wood.
They (NP) even stated this design was almost as good as the PFE design in testing. I would think that this testing was done under controlled conditions. I cannot imagine after a car is coupled a dozen times, and has felt the slack run in and out, it performed as well.
In the construction of early poured concrete houses, insulation was provided by soaking open pore wood in water and placing this wood in the middle of the wall structure before pouring the concrete in the thought that once the drying concrete absorbed the water in the wood, the wood would shrink, pull away from the concrete and provide a dead air insulating space in the middle of the wall. And this construction design was thought to be state of the art at the time.
Even today, our society is filled with people who when faced with facts, retreat into their beliefs cause they know those facts just cannot be right.