it's quite the opposite, considering that with say
it gives 10 lethality for 1100 gold, and consider that you can completely negate that armor pen and then some, for 300 gold
like unless you are level 18 then you are getting significantly more armor than they are negating with lethality --for a much lower price point.
But sure, unless you are stacking armor vs someone who is stacking damage, eventually the damage will outscale your tankiness but at least building some armor early will heavily stifle the mid game spike that lethality gives.