It's a 20% chance to learn the schematic each time you RE the item. That means you have an 80% chance to fail each time you RE the item.
Each RE is an independent event, so each time you RE is a new 80% chance of failing to learn a schematic. RNG will make you unhappy sometimes, and other times you'll get that shiny Dread Guard schematic on the first try. Yay random outcomes!
You obviously haven't even read the first post? The OP is making a claim that it is in fact a very very low probability that the actual RE-chance is 20%.