Sorry I'm no Ron Pattinson (though I have read and do admire and trust his work.) But I have done some historical reading of my own which clearly leads to my conclusion that names and fashions change in sometimes misleading ways. Is today's mild descended from 1850s porter? Of course not. But have the names mild and porter crossed paths and been used in different ways in different times and places? Absolutely. In turn of the 20th century brewing texts, mild refers to any beer not aged/vatted, and in some sources the terms stout and porter refer respectively to the stock and running versions of similar dark beers. So a porter would be referred to as mild -- possibly clouding the historical picture. Especially as 50 years earlier, porter itself was always a vatted beer, and stout referred only to a particular strength, even applied to pale ales. You have to understand the context in which terms have been used by various sources, so as not to assume that ANY beer ever called mild is in the same category and lineage. Ron is an outlier among writers in his attention to the evolution of particular lineages and ability not to be misled by current conceptions of style or popular usage.
EDIT Note that the distinction between "mild" and "stale, vatted, old, stock" goes back centuries. It is only after the decline of stock beers around the turn of the 20th century that mild could be exclusively applied to one style of beer, the one we know.