If you break it down and take the first part alone..
"I wonder if this isn't..."
and if you really get literal with it, I suppose the original assumption was that this wasn't going to be his last adventure but now the speaker thinks it may in fact be his last.
So..
Tintin: "This is not my last adventure,"
Some guy: "I wonder if this (really) isn't,"
But grammatically, its a 50/50 thing, either it 'is' or it 'isnt', so the language itself doesn't really have a preference one way or another. In passing, if I heard your suggestion (I wonder if it is) in natural conversation, I'd think it was a simple question. If I heard the original quote, (I wonder if it isn't) I'd feel there was a slight suggestion or innuendo. A little bit of wondering.