Several things can impact the training choices that high level athletes make. I do not have any particular knowledge about what these skiers you are referring to actually do for training. Some factors that MIGHT be influencing their choices:
1) When an athlete starts accumulate a very high average weekly volume, say in excess of 15-18 hours it becomes difficult to do all this on foot without undue fatigue and poor recovery. It could be that they are adding additional aerobic volume on the bike to increase the overall volume without beating themselves up too much. These pros have unlimited time to train so adding hours on the bike may make sense.
2) When athletes reach a very high level of fitness it become more and more difficult to extract additional speed gains from them. Coaches often introduce additional and unusual training stimuli into the program to try to get some improvement.
These are just 2 quick examples of what might be driving the decisions of these athletes. If you are world class and have been training for many years and seeking that last 1-2% then by all means you should try some different training methods that what you are used to. If however, you are an amateur who has limited time to train, have not developed the suggested foot born aerobic base then we feel you would be much better off doing the basics than copying what a world champion is doing.
Kilian and I discussed these types of things at length during the writing of the book. This book and most of what we write on our site is purposely NOT directed at professional athletes and instead is designed to help the amateurs who need the basics. It is a common mistake to try to mimic the training of a champion. For this reason we omitted talking about what Kilian does for training in our book. He, and probably the Italian skiers you mention, spent years doing the basics we describe in our book before they needed to worry about getting to complicated in their training.