It seems suspect to me, the sort of cod-etymology that gets propagated for decades once it gets into print by being uncritically cited wholesale and re-incorporated into a new work. Yes, porrum-->porro (modern Italian); but porrata (modern Italian) is a kind of leek quiche, and porridge has less to do with leeks and more to do with grain.

Leeks seem to have been carried northwards by the Romans and/or the French, but were already well-known; in Swedish they're purjolök, combining both words, the p-r form from Romance languages and the l-k or l-g forms from Germanic/Nordic.