Yay for apparent-possible-progress-pending-further-discusson+toward-agreement, then! But not to overpromise progress, he's worried for trope-based reasons that, after they have a few more days to mentally pursue this debate, they won't end up actually agreeing about the expected value of Greater Reality to people vs. "any living standards higher than literally Hell".
In particular, he's worried for trope-based reasons that the real heart of the disagreement between himself and Carissa will turn out to be that past-Keltham experienced his True Death and continuation elsewhere, and now remembers apparently direct confirmation of isekai-immortality being what truedying people actually experience. That evidence centers on his self-observation and his update off finding himself to be this self rather than somebody else: noticing himself to be somebody who remembers beginning in a simpler more real world and truedying there and continuing in a less real one. From the standpoint of somebody else watching past-Keltham get copied out of dath ilan, they haven't made that same observation, and it's fundamentally unshareable with them.
The famous central difficulty of writing a romance novel is finding a conflict that can't be resolved immediately through clear communication and emotional maturity; nobody wants to read a romance novel about silly people ignoring obvious solutions. Anthropically unshareable evidence is one way of providing a romantic obstacle like that.