It depends on what quality of potatoes you want for Christmas. The method you are using is the one method that will produce potatoes approximating to "new potatoes", which, of course, is precisely what people want if they are growing potatoes especially for Christmas.
The method he is advocating is pointless. Once the tops are cut off the plants the skins will set and the potatoes (apart from being badly slug chewed) will be no different in quality from the ordinary maincrop ones dug up in September and stored in the shed.
Personally, I prefer my Christmas potatoes properly roasted, which means 2nd earlies or maincrop out of the shed.
With regard to blighted topgrowth, I always remove this and flame gun the bed immediately to make sure that no live spores are left on the surface either to wash down or to infect the skins when the crop is lifted. Commercial Growers achieve the same result with sulphuric acid. When potatoes rot with blight in store it is usually because they have come into contact with bits of blight ridden debris on the soil surface
Anyway, why would you expect someone who works in a large garden centre to know anything about growing potatoes? They don`t grow things these days, they SELL things that other people grow
