This is a 'rational' quest. The most rational explanation is that Hazo hallucinated out of desperation. Uh oh. I legit did not even consider that it might not be true until now.
On the other hand, that portal we saw once that probably led to an afterlife was legit, so anything and everything we think we know about the afterlife must also be legit right? That's definitely how that works.
We saw Jiraiya use the Toad Thrust in the seance, something that Hazou didn't recognize and didn't remember seeing Jiraiya use before. When we replicated the movements to Naruto, he recognized it and confirmed that it was a taijutsu technique Jiraiya knew.
Outside of the dubious circumstance where Hazou saw the Toad Thrust, completely forgot about it, hallucinated it up during the seance, and then still completely failed to remember ever seeing it, the only reasonable explanation for how Hazou knew the correct movements to the Toad Thrust is that he wasn't hallucinating. Of course, it's still
possible that all that stuff I just said did happen, the forgetting and the hallucinating and all that, but we have no reason to believe that any of that happened.
And while the idea of a seance giving useful information may be very unlikely to us, it's far from beyond the pale in Marked for Death's world. We've seen the afterlife, we know it's a physical place where people who have died can move around and do things like that. Furthermore, when we first met Orochimaru and blacked out under the influence of his aura we saw visions of what was strongly implied to be genuinely possible futures if Hazou pursued apprenticeship with Orochimaru. Beyond that, we also have evidence just in general of eldritch beings who can reach into your head and affect your thoughts (thanks Mori Voice) or explicitly give visions to believers (like Hidan just told us). And that seance? We never told anyone, but we prayed to Jashin as it started. Jashin, who is explicitly capable of dumping visions into people's heads.
So overall it's not
that unlikely given what we know of Marked for Death's world, and I personally would be very surprised if Hazou had seen the Toad Thrust and then forgotten it and yadda yadda yadda, which means I do think it's pretty likely that we saw a genuine vision of Jiraiya during the seance. For completion's sake, it's also possible that we saw a genuine vision but the information within was fabricated, but I don't think there's any particular reason to believe this over the vision being genuine.
But hey, we'll find out ourselves once we get there.