Isn't the point in the documentation that it was not a problem due to VII cordite wear itself, but rather that 8z nitro wear did not give any gradual sign of range/accuracy drop off, and thus had to be given an arbitrary round limit per barrel (10,000 rnds)?
The MkVII wear in the MG barrel did not itself create a hazard, but by mixing round type and wear it thus negated the safety margin in the "10,000 rnd" limit for the 8z use, by upsetting the "round count".
IIRC MkVII was safe to use in MGs for much greater round count (typically 15-20k, depending upon rates) because the effect of wear on range/accuracy was a much more gradual process and easily observed by the firer.
Thus 8z was a better long range round, but it brought with it the hazard of abrupt change in performance once past a certain bore wear point.
IIRC the "burns the bore lead" aspect actually ended as soon as the original cordite mix was changed. Subsequent wear pattern differences between cordite and nitro loads are probably more to do with bullet shape and the way they set up into the rifling lead on firing.
The evidence from the MG side is that total bore wear is roughly the same for cordite and nitro, but that it is just distributed differently - cordite towards the rear, nitro towards the front.
Presumably the lower contact surface of the 8z boat tail is less of a "plug" at the rifling lead than the VII flat base bullet, and thus cause slightly less friction wear at that point.
Interestingly, there is one theoretical way to test this, which would be to fire a new rifle to burn out using VIIz ammunition - i.e. nitro with a flat base bullet. Sadly, there is probably not enough VIIz remaining to run the test; its the rarest type of ammo to find these days (all those pesky cadets shooting it off in the 80s!).
I had a No.4 barrel sectioned back in the day that had horrific throat wear - it was "washed" like a river-bed in the throat, which left grooves. Never seen that with NC propellant. Stupidly, it got thrown out in a house move in 2003... Wish I still had it.