I think it's better to ask "why did it take so long?" rather than come out with bold claims like "he's purposefully misleading viewers", because you're right, there is a lot of uncounted time here.

14500 gargoyles over 400 hours means Settled averaged one kill every ~100 seconds, but even at the start of the grind when his stats were at their lowest (86 attack and 91 strength), a gargoyle would take ~30 seconds to kill and that's without flicking the 15% prayers. That's over a minute of absence per kill - manually smashing, looting, alching and the occasional restoration of health/prayer cannot account for all that missing time.

I think the most logical explanation is Settled shifted from always flicking during the early portion of the grind, to a more semi-AFK approach later on by keeping melee overhead on with no offensives. This is reinforced by the decision for 72 prayer, which would make each trip last a whole extra minute without flicking, which alone would easily explain the absence in time.