investing in asteroid detection may not be the most important long-term thing, but it's at least plausibly related to x-risk (and would be confusing for it to be actively harmful), whereas third-world health has confusing long-term repercussions, but is definitely not directly related to x-risk.

I'm inclined to agree. A possible counterargument does come to mind, but I don't know how seriously to take it:

1. Global pandemics are an existential risk. (Even if they don't kill everyone, they might serve as civilizational defeaters that prevent us from escaping Earth or the solar system before something terminal obliterates humanity.)

2. Such a pandemic is much more likely to emerge and become a threat in less developed countries, because of worse general health and other conditions more conducive to disease transmission.

3. Funding health improvements in less developed countries would improve their level of general health and impede disease transmission.

4. From the above, investing in the health of less developed countries may well be related to x-risk.

5. Optional: asteroid detection, meanwhile, is mostly a solved problem.

Point 4 seems to follow from points 1-3. To me point 2 seems plausible; point 3 seems qualitatively correct, but I don't know whether it's quantitatively strong enough for the argument's conclusion to follow; and point 1 feels a bit strained. (I don't care so much about point 5 because you were just using asteroids as an easy example.)