It is not the weapon that will limit your XP per hour, it is the place you use to train. You can use the best equipment in the game but if you train on the wrong monster, you will not optimize your XP per hour.
Do you want to burn gp like it was going out of style? Do you mind no drops? Are you going for 100k xp or 1 mil xp? Do you want to maximize your overall XP per hour?
One of the very underutilized tools in the game are things like super strs and atts, even normal increase you stats +12%, so using them will give you an added 10% XP per hour. Fighting next to an alter, like on monkey isl, and praying ultimate str and att prayers will also increase your XP per hour by 15%. No weapon offers a 25% better XP per hour like the other related things would, well unless you were trying to compare training with a rune weapon and a drag weapon, but that is beside the point.
You fail to mention that if you were using an iron sword that you'd gain barely any xp..
You make a very good point but this only works if used with a decent weapon..