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.
I read that and i said i disagree, because if you have 100K ore in 4 tiles, you can place 1 miner that covers it all, you add 2 PM3 you expect 120K ore, now you add another miner and the miner overlap, you add another 2 PM3 and you still have 120K ore, you add another miner that makes it 3, all ovelapping on those 4 tile which is possible in the game, you add another 2 PM3 for a total of 6 and you still have 120K ore you can even add a 4rth miner, using a total of 8 PM3 and you'd still have 120K ore.
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.