Jack, events in 20th century Kenya don't tell us useful information about medieval Europe, neither does Turkish music inform us about 14th century France. In the link you give for Douce Dame Jolie the dorian mode is transposed upwards, so the transposed reciting note is d'. I find it difficult to understand why you can't see what a central role that note plays in the melody.

As Anne says, medieval writers were the literate few, and so they are the only audience who can give us information.