This is all nonsense! Unless the rim is actually designed not to be with used with rim brakes, it doesn't matter whether the sidewalls are machined or not. If you use rim brakes on a rim with non-silver anodised sidewalls, the coloured anodising on the sidewallls will wear away, it's normal.
As already said, machined rims are a relatively recent thing.
This is all nonsense! Unless the rim is actually designed not to be with used with rim brakes, it doesn't matter whether the sidewalls are machined or not. If you use rim brakes on a rim with non-silver anodised sidewalls, the coloured anodising on the sidewallls will wear away, it's normal.
As already said, machined rims are a relatively recent thing.