Actually I've timed each inventory with both axe's: Rune and Dragon. I can safely say that even though you can get an inventory of willows faster, it isn't the fastest method. Here's my findings:
rune axe:
willows = exp per inv 37800, 2:14 (Time to get a full inventory) - 282 xp per scond
yews = exp per inv 98000, 4:14 (Time to get a full inventory) - 385 xp per second
magic = exp per inv 140000, 5:35 (Time to get a full inventory) - 417 xp per second
dragon axe:
willows = exp per inv 37800, exp per inv 1:37 (Time to get a full inventory) - 389 xp per second
yews = exp per inv 98000, 4:06 (Time to get a full inventory) - 398 xp per second
magic = exp per inv 140000, 4:37 (Time to get a full inventory) - 505 xp per second
You might wanna update your guide bro
Hovaldo