They can buy a Tabi and Aegis/Glacial shroud or have parts of randuins before you ever get an apen item.
Apen items aren't needed until they stack armor well past 200. Most of the reduction comes from abilities, which is free.
As for stacking armor beyond 300, that only serves to waste more gold because the armor is reduced by percentage so it scales without limit. If Malphite had 600 armor then these champs could reduce it and penetrate it to 142 armor with only purchasing Last Whisper. That's only 820g spent on penetration (after subtracting the value of 40AD from LW). This is why I believe it is not cost effective to stack armor against this team comp.
Someone mentioned the slow on FH and Randuin's. That's a good point--it would hurt Xin in particular. Xin may need to build beefier in response--instead of stacking AS in spite of the enemy FH and Randuin's.
As for Thornmail, seems like a waste of 2000g since it's useless without armor and both Urgot and Wukong do a significant portion of damage from abilities. With effective armor reduced to ~50, thornmail is not a good item to buy--not even against auto-attackers.
But this is just theory. I want to know how it works in practice before scrounging a team together (takes forever to get my friends moving). Can they keep the armor reduced in a fight--in real practice? That's why I'm asking if anyone has used this.
I've heard of similar strategies for Dominion (Sauron's guide: "Other team compositions that I've seen work well: Four physicals top lane (at least one ranged) that all rush LW and Ghostblade").
If it works on Dominion, someone has probably at least tried it on TT. Did it work?