There seems to be a lot of context missing. If Rammus never attacked him, do you mean with an AA or with no abilities? Did the Kogmaw just go from full to dead literally just attacking Rammus with Rammus thornmail ability on? That I find doubtful especially from full. I fyou mean Kogmaw had like 100 hp left, and attacked Rammus a few times with W up then it makes sense. That's just bad play not OP Rammus. If you mean he combo'd out the kogmaw, then you must think any mage is OP because they never have to AA them.
Rammus full combo at level 6 is like 510 HP (assuming you attacked once during taunt and his W was up) with no AA's. 646 total if you include Cinderhulk if he has it completed by lvl 6. That's all pre mitigation. However, that's assumign he just sits in everything over 8 seconds. Kogmaw has over 800HP at lvl 5.
Seems like a lot of information is intentionally left out for the sake of hyperbole.