I re-read the chapter again and again, especially the part when Chopper was mentioning Ace and healing Luffy.
Put it into this context:
Chopper: "To think a thing to happen to Luffy's brother" is referring to the first newspaper which was the execution of Ace, So Chopper is still clueless about whether Luffy was in the war or not, just that Ace will be executed in the war.
Chopper: "I want to heal his wounds, I'm sure his gone overboard again" is referring to the battle before Chopper turn monster point and is clueless to what happen to the crew, still thinking that they are still on SA after fighting.
Now, when the narrator mention Usoop Island, there was no mention of "turning back the clock" so we can say it's within the current time line, so when Usoop read the newspaper, he new luffy was in the war and that Ace died in front of him.
The bird coo than flew to more island and than the narrator mention Chopper's island, which was Introduce as "meanwhile on the island…" so you can say that as Usoop was mentioning how he wants to leave, is within the current time with Chopper wanting to go back to the crew, lets say the bird coo drop off the newspaper to usoop island first, a few more islands and than arriving as Chopper left.
Now look at what Chopper said when he saw the newspaper.
Chopper: "Luffy, why again" can be refer to the mentioning of Luffy in the war and that he is in critical condition, thus why Chopper said that. So I believe it's not a bounty or anything, just plain and simple. I hope this make sense i was speed typing ahahaha
By the way Chopper looks too happy when he was talking to the birds and humans that is why I believed he did not know about the situation, until he read the latest newspaper, that is when his reaction was similar to Usoop.