I haven't said all armour items were gutted, in fact I think some of them are pretty decent like thornmail and randuins, and to a lesser extent frozen heart. However, when you concider the amount of armour in the game has increased, including the fact that %pen/shred has decreased, many % pen/shred items end up reducing about as much armour as they had previously, except now on targets with a smaller health pool to back up those reduced resists.
So while armour items may be better against physical damage, the pen items in the game make up for the difference, leading targets to be just about as tanky as before the update, in addition to lowered damage reduction from the opposing damage type, leading to taks having an even weaker earlygame than before with virtually no compensation lategame.
Even worse is that in riots words, they wanted to alow tank builds to become more effective through game time, and yet they still push the idea that adcs are the only class capable of lategame functionality. Thus their point on power growth is moot and leads to tanks just being weaker overall.
This isnt just anecdotal as well, every single tank that wasn't directly touched by riot after the tank update suffered a MASSIVE winrate drop and some still haven't fully recovered despite direct and indirect buffs.
We also STILL havent looked at the %pen/shred innately in champion kits, which has inadvertently made some champions significantly better than others, and likely is the only reason why juggernauts like darius and garen remain playable in this meta.
So, from what i understand, you claim resistance items are better at reducing damage from their respective damage types and worse at the opposite making tank builds less powerful after one item and more after several. Thus, health gained from items or other sources must be more effective when combined with these increased resists against their corresponding damage type.
I claim that this causes problems due to the penetration items in game nulifying this effectiveness while still having reduced effectiveness against the opposing type. Additionally this causes scaling problems as tanks have difficulty fighting in non-singular damage comps until they have completed at least 2 items, much like adcs and other classes, and yet dont scale nearly as well leaving them feeling underwhelmed at all stages in the game.
Though his point may be misleading, it shows a general truth about the state of tank items in that they are almost universally weaker than they were before the update no matter what stage in the game you are.
The only thing really propping up tanks, which is also being abused by another class it wasnt meant for, is stoneplate.
We can discuss that further if you would like, but im feeling pretty done on this topic for now.