Thundering typhoons! Welcome aboard the forums, BommenEnGranaten!
Red Rackham's treasure was buried on the island, perhaps, and Sir Francis found it during the two years he was there, or he (or one of the native islanders he befriended, swam down and collected the treasure from the sea bed where it ended up after the explosion?
Given the almost obsessive amount of attention Hergé paid to the tiniest details, I can't imagine he just overlooked this.
Hergé may have been a meticulous artist, but he was haphazard with details of plot at best - I mean, look at poor Barnaby in this story: he's shot on Tintin's doorstep, and then not heard of again, until eight pages from the end of Secret of 'The Unicorn'
Haddock finally tells Tintin that "the little birds man" survived, and he doesn't even get a proper name until more or less two pages later (or a week of strips in the serial version), when Max Bird explains about what Barnaby was doing, and why they had shot him, in a massive splurge of plot exposition that fills frame after frame with text, as Hergé rushes to tie up the story before he runs out of book...
And in the case of the treasure, I'm not sure it actually matters that much...
The treasure is what Alfred Hitchcock called "a McGuffin" - the item initially presented as the objective of an adventure, but which in effect only exists to allow the adventure to happen.
One of the most famous "McGuffins" (I know it's not a Hitchcock, but the principle's the same) is Dashiell Hammett's The Maltese Falcon
- it really plays little or no part in what goes on, and even when the pursuers get their hands on it, they find it's actually a fake!
Even then, undaunted, they immediately set off again, on yet another chase, in full knowledge that it's the idea of the object and not the Falcon itself that drives them...
Hergé needed something to have the crew get involved with the Bird Brothers, then set off around the globe on an escapade, and the mythical treasure served the purpose. How it was obtained was irelevant to Hergé, he just needed a goal for those involved in the quest.