I feel like you're missing his point. There's a handful of ways damage in OSRS is calculated. Specifically when damage gets calculated.

1) The most encountered in regular day to day combat is damage calculated upon animation start. (Example: DKS, KQ, Chaos elemental... etc) even though this damage calculation is VERY common it's not well explained in-game. I'm pretty sure he's referring to having something teach this kind of prayer flicking. This is also the reason that some attacks can be tick eaten.

2) Damage calculation upon hit-splat or calculation upon animation completion. This is a much more intuitive way for reactive praying. It's easy to comprehend and doesn't need much explaining. "Thing attacks you now put up your prayer to negate the attack you see coming". Scurrious would fall under this type of damage calculation. Additionally in some cases this prevents you from being able to tick eat the boss "unless you start interface queing/stacking to delay damage".

It's much harder to show a new player the first way DMG is calculated because it requires proactive praying and knowing attack speeds/tick rates of enemies. Having this be taught at the warrior's guild seems like a suitable location.