Personally, I highly doubt that to be Holden's true rejection, though it is most likely one of the emotional considerations that cannot be ignored in a strategic perspective. Holden claims to have gone through most of the relevant LessWrong sequence and SIAI public presentation material, which makes the likelihood of a deceptive (or self-deceptive) argumentation lower, I believe.

No, what I believe to be the real issue is that Holden and (Most of SIAI) have disagreements over many specific claims used to justify broader claims - if the specific claims are granted in principle, both seem to generally agree in good bayesian fashion on the broader or more general claim. Much of the disagreements on those specifics also appears to stem from different priors in ethical and moral values, as well as differences in their evaluations and models of human population behaviors and specific (but often unspecified) "best guess" probabilities.

For a generalized example, one strong claim for existential risk being optimal effort is that even a minimal decrease in risk provides immense expected value simply from the sheer magnitude of what could most likely be achieved by humanity throughout the rest of its course of existence. Many experts and scientists outright reject this on the grounds that "future, intangible, merely hypothetical other humans" should not be assigned value on the same order-of-magnitude as current humans, or even one order of magnitude lower.