I think you are right: the chances aren't extreme, but are non-trivial. This looks to be a chart that has the mortality data necessary to answer your question (although presumably it also includes "non-natural" causes like suicide and homicide): https://www.ssa.gov/oact/STATS/table4c6.html. The first two columns show age, and the probability that a holder of an American Social Security card who is of that age will die within the next year. For all but the oldest ages, I think it should be accurate to simply divide this by 365 to calculate the chance that an individual of that age will die within 24 hours.

From the chart --- and oversimplifying HN demographics by assuming they match those of chart and consist only of 30-year-old American males --- the probability of death within the year is 0.0015; making each individual's daily chance of death 0.000004, which is about 1 out 250,000. Unless I'm misremembering, to calculate the chance that at least one person out of a group of size N dies, it's easiest to exponentiate the probability of survival (1 - .000004 == 0.999996) ^ N, and then subtract this from 1 to find the chance of at least one death.

If we guess that we readers are one-thousand 30-year-old males, I think that means there is about a half a percent chance that one of us will die before tomorrow ((1 - (0.999996 ^ 1000)) == 0.004). If we instead assume ten-thousand 30-year-old males, then we get about a 4% chance that someone won't be around after tomorrow. If we generously assume a hundred-thousand such readers, then there is about a 30% chance that one of us won't make it another full day. I don't know what the actual readership numbers are for this post (or maybe the grandparent was self-referencing their own comment rather than the main post?) but it seems likely that it's somewhere within that range.

If we use a more realistic age distribution for HN, the probability would go up (older readers increase the probability more than younger readers decrease it). On the other hand, if we assume that HN readers on average have better health care and less risk of violence than randomly chosen Social Security card holders, then the probability would go down. But suicide risk would probably go the other way, so I don't know what the total correction factor would be. Still, I'd guess this estimate would remain in the ballpark. Corrections to my methodology or calculations appreciated.