That's a bizarre definition:

Cambridge English dictionary: "animals from the sea that can be eaten, especially fish or sea creatures with shells"

Merriam Webster: "edible marine fish and shellfish"

These are much more reasonable definitions, more line with modern usage. I doubt you will find any dictionaries that mirror your Britania definition.

I grew up in and have lived near the land of 10,000 lakes (MN) almost my entire life and not once in my 35+ years of life has anyone referred to fish out of those lakes as seafood. Ever. In fact people around here I feel would be utterly confused if you did. I wouldn't fault him with not following Encyclopedia Britannica exactly.

Huh? In MN, WI and the upper peninsula of Michigan I have sat in multiple restaurants that had a seafood section in the menu that proudly claimed fresh caught (insert nearest great lake) fish. Maybe not so much in Central Minnesota, but I have definitely seen this in Duluth, and I doubt that the average Minnesotan would be confused by calling lake fish seafood. Although if we are being truly pedantic I suppose that the great lakes would more properly be considered inland seas.

Totally agree, but GP may not be a native speaker.