this is not true. lethality is ineffective against high armor.
Assuming you are a tank since you are building thormail - lets say you have 70 base armor when you get the item. The thornmail gives you 80 bonus armor, so thats 150 total armor from just that item.
The current formula, I believe, from https://lolmath.net/articles/league_of_legends_formulas.html, is Armor/100+armor as damage reduction.
So, in this case, it would be 150/250 = 60% damage reduction.
Duskblade would make it = 129/100+129 = 56% damage reduction.
So, your not losing 26% of the benefit but just 4%.
The lethality builder is being penalized for your armor building by just having 55 AD and the damage passive against you. Your armor is also reducing his 55 AD worth of damage significantly.
You are also forgetting the 250 health you get from thornmail which makes your total armor even more effective.
I'd say its effective enough. But, there are some broken champs that do way too much damage anyway as part of their kits and unless you have multiple tank items and some damage reduction in your kit, they can still 1 shot you at even gold. That has more to do with champion design than items.
Also, lethality is completely countered by one amazing and cheap item - seekers armguard. The duskblade at 2900 gold negates 21 armor, and a fully stacked armguard for 1100 gold gives you 45 armor and 35 AP. You are entirely negating the 21 armor pen and having an additional 24 armor to deal with base damages/ad/55 ad from the item for just 1100 gold.
Id argue it is lethality that is completely countered by cheap armor items, not the other way around.
This is why, if I see even a single lethality item on a snowballing enemy champ, or the enemy Miss fortune building lethality, even if I am playing some squishy champ or am the opposing ADC, I pick up a cloth armor maybe even a chain vest if i can afford to spend the gold on it, completely negating all her lethality. My IE + zeal + chain vest = 5200 gold gives me 70 AD, 40% crit chance, the crit passive, 5% MS and 15% AS and the enemy MF has spent 2900x2 = 5800 which is 600 gold more than me, but all she has is 110 AD which, after negating 18+21 = 39 of my chain vest, is pretty much all she has to fight me with. My chain vest will eventually become a GA too, but her lethality will never affect me.
Also, TBC armor pen is a stacking passive after applying damage. Its not efficient on burst AD assassins, most get it for the survivability and to duel other tanks/bruisers than for burst.
Its good on lee, for example, who Q you (4% armor shred) q again - 8% armor shred total, E, 12% armor shred all within a few seconds. Big advantage is it doesnt affect only his damage like lethality does, all AD damage from any source on the target is increased. But, in itself, phage passive isn't so useful on burst assassins, especially if they have already built youmus, 20% cdr is also not relevant on them during that particular kill attempt, and by the time they shred any really meaningful amount of armor on low armor targets, they are done with their rotation and need to get out or face dying themselves. for instance, caitlin, at 88 armor without GA level 18, is reducing 88/188 % physical damage = 46% physical damage. Lets say the enemy zed gets 2 shurikens, one auto, and two slashes on cait before exiting the kill attempt. he starts with R onto cait, w, e, q, then R to exit. The W is double, 8% armor shred, but since they happen simultaneously they shred 8% armor right away but havent shredded any armor before, so next the 2 shurikens receive the 8% damage reduction and add another 8% armor shred = 16%. The R pop basically gets to do just 4% more damage despite 4 stacks of cleaver armor shred, and the two shurikens before it gain even less bonus damage from the 2 stacks of armour shred.
Its Arpen to be effective against tanks/juggernaughts and lethality against squishies. So, the tradeoff of building lethality is doing very little damage to tanks (and can be entirely negated by marksman core item - GA, or mage core item, zhonyas). The tradeoff to % pen and armor shred like TBC is doing little to no extra damage to squishies.
All in all, lethality is heavily countered by anyone that matters. Its actually most effective against bruisers that build only offensive health/damage items like Irelia and renekton, or against ADCs (who do have their sp to peel and arnt inherently laning against the assassin) until they get their GA when all the lethality gets completely negated.
Mid can rush seekers after first back and negate even 2 of your lethality items without upgrading to zhonya.
are way too easy to build. Even though they cost 2900, the 350$ 
850$ 
and adcs 1300$ 
875$ 
, which DO NOT have an implicit killing-reward nature, have to deal with the expansive builds). Needless to say, CDR should be a tradeoff to damage.
also have pretty decent stats, and the same easy path issue. Getting tankier should be a tradeoff for Damage. But the armorpen passive of the the item, summed up to the (again) non-tradeoff 20% free CDR stats makes it an obvious, easy-taken choice.
have low mana costs, no mana at all or are junglers with high mp recuperation stats due to bluebuff/smite manaregen passive)
is a must, obvious 1rst item choice for mages, but it's onlye one item and not half of the buildpath.
/
and Duskblade is a MUST HAVE on almost any champ that builds lethality. i think there should be more lethality item diversity
