Because if you had stable amount of population if people lived to be 100 years old then each year 1000 out of 100k would die.

In this case, I think the assumption that doesn't bear out is "stable amount of population". The relationship between death rate and expected lifespan is likely calculable, but only if you also have either age data or at least approximate change in birthrate data, preferably going back to living memory. We might have that available somewhere, but I don't think your estimate came from having it.

Like let's say there's a terrible war, and half of all people die. Can we conclude that most people are dying around 2? This example is extreme, but shows that we at least need some averages over time to start making a case for life expectancy being predicted well by the data

Yes, but also part of my point, that the above author based on these numbers implied that World is getting more dangerous, but firstly as you mentioned these numbers are meaningless if we don't know the exact demographics of the 100k and secondly for a stable 100k these would be really good numbers.

Makes sense from that perspective. Thanks for taking the time to clarify