So, just a major IV chord, AKA "IV." I could have guessed he meant, because Dorian on A would have an F#, that the four chord would be D (major).

But he also refers to an "Aeolian VI."

So would that be F (major) according to this notion of modal chords.

Yet having already thought through the possibility of that meaning (i.e. building all triads on the notes of the respective "modes") before asking the question, I didn't recall any modern renditions of Wellerman that use an F chord in the second line.

Hence the confusion.