I think that's why ReiMakoto said you agree but are just arguing over word choice lol. Schrodinger's armor lol :P. I was simply trying to say because there exists a situation of diminishing returns, mathematically, they're not wrong in attempting to apply the word usage. Though I understand you're suggesting is it 'really' diminishing returns when in one example you'll die and the other you live because you can't achieve the required eHP by one spell alone? Is living a diminishing return? What if you used every ounce of that eHP so none of it was wasted? But that's where he's describing the math of DR% and you're describing playing the game and subjectively it's not diminishing return if you either get nothing (dead) or get everything (live)- but of course is not the only situation players will find themselves in.
Also not really for you but I had made it and figured it might help someone (why spreading can be better than stacking, even though stacking produces much higher eHP):
We have two spells that block 50% damage for the next 5 attacks (using attacks rather than time for simplicity).
Now we have a monster that'll attack 10 times, each attack dealing 100 damage.
100 , 100 , 100 , 100 , 100, 100 , 100 , 100 , 100 , 100
We'll run two options, a. spread out the defensive cooldowns (50% twice, or half of 100 = 50), and b. run them together (50% twice at the same time = half of 100 is 50 and half of 50 is 25, so 75 damage out of 100 reduced or in other words 75% reduction):
a) (50 50 50 50 50 50 50 50 50 50) = 50*10 = 500 total damage
b) (25 25 25 25 25 100 100 100 100 100) = 25*5 + 100*5 = 625 total damage
You took more damage when you stacked them. You took less when you spread it out.
Because games are complicated it wont always be this case, like as you're talking about tank busters. Using both cooldowns may mean the difference of life and death.