were the ones in which Fujitora and Sengoku were onboard? Jack is so OP that not even 2 Admiral level fighters can stop him! And then when he was about to take Doflamingo, he was attacked by the remaining 2 Battleship's and was forced to retreat because he was too weak due to the wounds which he got from Fuji/Sengoku. And then fell into the ocean and was "presumed dead" but was saved by plot because one can't just offpanel an island conquerer and an animal tamer.
Your own reasoning collapses on itself. Sinking the battleships was the easy part, it were the admiral and former fleet admiral that were the issue here. If Jack truly had beaten the two of them, it would not be "He sunk two battleships and was presumed dead" but "he sank four battleships, killed 2 admirals and rescued DD". The fact that he was only able to sink two of them and consequently was presumed to be dead, pretty much means he got served by the admirals to the point they assume he has to be dead. Jack is an irrational lunatic, he attacked a convoy with two admiral present head on without a plan. That's really not a guy who's going to stop after he was able to kill two admirals. Especially when sinking four battleships really isn't that big of a deal at this point in the story. This is something several characters could accomplish with ease if it were just the battleships. The fact that Jack was only able to sink two of them and was then presumed to be dead, shows he pretty much is all brawl, no brain and got served.
I really don't get how you could be so positive about it and how you're interpreting your own reasoning in such a contradictory manner.