No this is not true, it's not 20% more ore for every miner you equip with module. Maybe your thought do not correspond to your wording.
You do not get the same amount of extra ore from module, not if the miner overlap. The first one adds 20%, then the other add nothing at all in the previous example. Which is another diminishing return mechanism.
I never said it was insane i'm just saying that there is no relation between the amount of ressources modules gives you for free and the amount of ressource you decide to divert for their production. If you divert all your ressource to module production, you still consume ressources, in no way the generalization that modules are built using ressources bonus from modules is correct. It can be sometimes in particular cases.
The actual bonus given per modules depend on the richness of the ore patch, considering mining drills do not overlap, each mining drill covers up to 12 tiles of ore. If the richness of the ore is low, like near the middle of the map, then 12 tiles can contain as low as 100 ore for example with 10 ore per tile, and if you build the same mining drill far away from the middle of the map, and you decide to put prod module in the mining drill, then it's possible that the same mining drill is covering 12 tiles worth 1.2 million ore. You get a different amout of bonus ore per module compared to its own cost. Sometimes they pay off for themselves by a lot sometimes not at all.