Can we avoid dragging realism into this argument, please?
D&D isn't a pvp game, so there's no point comparing a rapierist and a fully armored, knight style guy, because ideally they're not going to face each other: they're going to face goblins, orcs and hobgoblins, against whom heavy armor fares just fine, and they're also going to face hill giants, dragons, gnolls, titans and a ton other creatures that are just so big that they can just snap your neck or trample you regardless of how much steel you're wearing, monsters that an actual heavy armored dude never faced irl.
So, while realism works fine as a benchmark if we consider a fight between human sized combatants, I really doubt that it's applicable wholesale to your average D&D campaign. Also, I'm pretty sure the guy whose post you linked never faced a dex 18 ( or dex 20 ) rapierist, so we'll never know who has the better AC irl
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