I wouldn't say well founded because the advice has turned around 180 degrees in just a decade. The study says you can reduce the risk by introducing at an early age, the link to restriction is unproven as far as I know.I believe that the theory that the growth in the number of people with allergies is strongly connected to the modern tendency to restrict what we give young children is very well-founded and has been around for a long time.
Also peanut allergy is hardly ever considered alone but normally alongside asthma, eczema and other intolerances. This study and others like it, are promising but I don't think it's anywhere close to understood yet. I think that's also because there's no one single explanation. There might be something in the restriction theory, there might be something in the spread of peanut oil as a base product, there might be genetic factors, there might be environmental factors. You might solve one thing, peanuts but not solve something else, like asthma. So is it modern life. Well it could well be but not just one factor.