I agree. I was assuming (I know!) that PGM would use the correct spelling of chords, thus Am would be 1-b3-5. Relative minors are never 6-1-3.
I think I disagree with that, probably because I see a different context.
In the key of C, the Am chord is the minor sixth of C, so 6,1,3 (6m, vi).
I agree entirely that in the relative minor
key of Am the chord should be spelled 1,b3, 5 (1m, i).