never heard that advice before. It certainly wasn't the case with ours! Clay is clay no matter how many potatoes you cram in!
That is also true.
Newly prepared ground that was previously predominantly grass is also risky for spuds due to the likely presence of wireworm, which can be a serious pest for potatoes.
One possible benefit could be the quite extensive leaf canopy produced by spuds which (along with the earthing up) tends to supress weed growth. Doesn't actually remove the weeds though!