Exactly. He's stuck arguing from a conditional universe in which he must exist, so he observing his existence is a foregone conclusion under any hypothesis about how he came to exist. Hence his observing his own existence cannot discriminate between any hypotheses about how he came to exist.
Of course he exists if he uses himself as evidence. Just like any other evidence. All evidence exists in a similar conditional probability space. It has to exist before you can use it.
Not to worry. Jabba can only attempt to use evidence he doesn't have if the evidence is
not him. So it all works out nicely.
The following numbered items are foregone conclusions:
1. If H* is true, the prior odds would have been stacked sky high against Jabba ever being around to argue against it.
2. If H is not true, his existence would invariably have him arguing correctly against H. His nonexistence would have him not making any mistakes either. So no way to go wrong here.
3. Jabba would only argue incorrectly if both H is true and Jabba miraculously beats the giganogargantuan odds H stacks against him. Clearly, (3.) needs a lot of help from (2.), probability-wise, to stay in the ball game. (3.) needs to be as likely as (2.) just to break even. I wouldn't bet on it.
*When I say "H", I mean an interpretation of broadly accepted science, not the science itself. As should ~H be, IMO.
None of which requires Jabba to be immortal (though it might help).
But a "fallacy" which might well* have Jabba more likely to argue correctly than incorrectly against H strikes me as an odd kind of fallacy. Give me a fallacy like that, and I'll use it.
Where I think Jabba went wrong is when he tried to advance an alternative to H that was based on nothing but anecdotes. I don't think the formula is the problem.
*And I only say "might well" because I have no intention of going through the hassle of advancing one of the more obvious alternative interpretations of the science which would play better than H + Jabba. Partly because I fear the trademark stubborn, troll-like, mundanity-clutching opposition would quickly get on my last nerve. And partly because it's Jabba's job to advance a playable alternative.
All I'm interested in is studying the pathological denial of probability.