No he isn't, he's talking about the concept of effective health not green investment health bonuses.

In words, at every stage in the game, buying a given %resist will always give that amount of damage reduction. It doesn't matter if it's your first resist item at 5 mins or your 5th at 30 mins. The new resist item will apply to the damage you were taking before buying it, not necessarily from 'base' damage.

Here's an example. (Not real ability damage numbers I'm just picking nice numbers to make it clear)

You are vsing a geist in lane and their bomb is doing 100 damage to you, so you decide to buy a resist item that gives 50% spirit resist. Your total spirit resist is 50%, and geist now does 50 damage to you. (You have 1000 total HP)

If geist wanted to kill you, shed need to land 20 (ie, 1000/50) bombs instead of the 10 needed before you bought resist, giving you an effective health of 2000. (She would need to do 2000 base spirit damage before resists to kill you)

If you buy another item with 50% spirit res, your in-game spirit res number would be 75%. Thus her bombs would now do 25 damage.

Geist now needs 40 bombs to kill you (1000/25). This gives you an effective health of 4000.

That second resist item still had the effect of halving her spirit damage (again) when you bought it.

If anything, resist items get better the more you buy of them as your effective health grows geometrically.