I think they do mean that the layout of the road means you can’t drive at the speed limit. In the UK we have quite a lot of rural roads that are technically legally 60mph limit roads, but they are in reality very narrow, windy roads that you couldn’t safely drive on at 60mph. I guess CoMaps goes by speed limit for estimates so it measures these roads as if they are 60mph but in reality you may only be able to go 20mph without dying