Without reading the article (site is down) my expectation is that the data will show significantly lower salaries in Europe than in the USA. One of the explanations is that there are almost no genuine software development companies in Europe. Software jobs there are mostly internal (enterprise) software development or customization, and not software product or service development. Being a cost centre instead of a pro…
While the valley definitely skews the high end in America, it employs a fraction of the total developers in the US. There's the other 90% of the country that doesn't live in the valley that works in those very same "corporate development" jobs that are in Europe. You don't think companies like Walmart, State Farm, Allstate, Epic*, Target, Best Buy, UHG, Dell*, IBM* etc. etc. employ boatloads of developers all outside of the bay area? They should similarly bring down the average pay.
It's also unfair to say Europe doesn't have development jobs... heck SAP is the third largest software producer in the world and they're based in Europe. SuSE is still headquartered there. I have no doubt there are plenty of startups there as well.
*for accuracy - Epic, IBM, and Dell are all at least partially software companies but employ a ton of people outside of the bay area