You all are wrong about the armor calculations. Doesn't change the fact that his point is true though about how armor scales, every point is as equally valuable as the last. The question is if it may be more worth it to get more HP after a point, which is a different matter.
Armor scales like this.
Every point of armor/mr grants you +1% effective HP vs that damage type.
At 100 armor, you are having your effective HP pool doubled vs physical damage. At 200 armor, you had it tripled.
Lets use a fairly easy example.
You have exactly 2000 HP. You have 100 armor. This is worth 4000 HP vs physical damage.
You can buy a Warmog, or a Frozen Heart.
Warmog gives you 2800 HP, and still 100 armor. This is 5600 HP vs physical.
Frozen Heart gives you 2000 HP, and 190 armor. This is 5800 HP vs physical damage.
Now lets change the story up a bit. You have 3000 HP, and 200 armor. This is 9000 HP vs physical.
Warmog brings you to 3800 HP, and 200 armor. 11400 hp vs physical.
Frozen Heart gets you to 3000 hp and 290 armor. 11700 hp vs physical.
Armor has still had a greater impact on your durability vs physical than HP would have. The main issue is you are rarely taking pure one damage type. Pretty much the moment you start taking any other type of damage instead of just physical, HP's effective value on your durability is greater than armor. Although more HP leaves you as a bigger health battery for LS users, so it isn't a clear cut tradeoff all the time.
Now, lets pretend you are Rammus, and are vs a pure physical team comp. You build up to 500 armor while W is on, although you only have 2500 HP. What will bulk you up the most? 500 more HP, or 100 more armor?
3000x6=18000 is the HP case. 2500x7=17500 is the armor. You gained more HP from a much lower gold value of HP than armor just because you already had such a big armor "multiplier."