This is another case of readers letting the weekly chapter format affect their critical thinking. Even less than that, critiquing the quality of the story through spoilers which don't convey the sequence of events Oda intended for readers to go through. We'll learn how Jinbe escaped Linlin safely later. Right now, the focus is on Jinbe's return, which is clearly meant for a end of chapter cliffhanger. How he survived, how did he escape the BMP? That's how you build intrigue.