So who is this economist that the Wall Street Journal thinks is on its side? Well, economists do change their styles and their views as they get older; Mundell changed more than most. Those seminal early papers were crisp and minimalist; they looked forward with remarkable prescience to the wild and woolly, out-of-control world of modern international macroeconomics. By contrast, Mundell's writings since the early '70s have been discursive, one might almost say rambling, and often reveal a sort of hankering for the lost certainties of the gold standard. (And yes, he has said a few things that can, with some effort, be construed as support for supply-side economics.) The precocious theorist anticipated the 1990s; the elder statesman has hearkened back to the 1890s.
So you can take your pick as to which Mundell you prefer; but the Nobel committee basically honored Mundell the younger, the economist who was iconoclastic enough to imagine that Canada, of all places, was the economy of the future--and was right.