But if samsung drops production 10%, and micron increases by the same amount (in volume, not percentage), prices and profit margins would remain the same, but micron would bring in more profit, while also denying your competitor (samsung) the extra profits.
This move by samsung is a gamble. If other DRAM makers all think like samsung does, then all is good. But if any of them get the bright idea to screw over samsung hard with a flood of dram supply at inflated prices, samsung will have a bad time indeed.
Your AMD/nvidia comparison is flawed because in that case, you have the makers tied to TSMC, which places a natural limit on supply. DRAM makers have applied entirely artifical limits on production right now, there is nothing to prevent these companies from slightly ramping up and stealing that extra profit right out from under samsung. By doing so, they woul still get those extra profits, but also deny samsung profit in the process.
In capitalism you dont let your competitor get an easy break. AMD did that with polaris and pascal, and it ended up with nvidia making a mint in extra profit, which is gonna bite AMD hard over the next few years.