You don't even need to go back to old english: per your source, definition 1 under english etymology 1 is:
(chiefly Britain, uncountable) Any cereal plant grown for its grain, or the grain thereof.
edit: but yes, in the context of discussing a poem from the 12th century, old English would make more sense :) Interesting that the translation is from the french ble , which I would intuitively associate with the modern french blé meaning[1] wheat
Corn originally meant “small seed”, and then was generically used to refer to what are now typically called grains or cereals (wheat, oats, rye, ...).
The new-world grain maize, which had no existing name in English, was called “Indian corn”. Later, that was shortened to just “corn”, which eventually resulted in the generic word “corn” losing currency in North America.