Yes, the book says the 1918 flu hit healthy robust people the hardest. There are many other differences. For example, that flu had a super short incubation time, so it swept through communities in 7 weeks, and military bases in 4-5. On many bases thousands could become ill in a single day. It's spread was much more rapid.
But there are lots of other similarities as well: Hospitals were rapidly overwhelmed. With both flu and coronavirus, it is the pneumonia that is dangerous (also, apparently true of measles). Some people died very quickly of their own immune response and massive lung damage (SARDs, severe acute respiratory distress) while others succumbed later to secondary bacterial pneumonia. Health authorities were reluctant to acknowledge the problem for fear of creating panic. Many people and communities attempted to self-isolate, with very limited success.
There are some possible similarities that have not yet played out - notably multiple waves of infection with different mortalities and characteristics.
Overall, the book has a lot of detailed scientific explanations that I enjoyed. One of my favorite parts was a detailed explanation of the significance of "H" and "N" in flu names (e.g. H1N1), and why flu mutates so rapidly, and is so likely to cross over in certain animals. Overall, I recommend it if you like detailed but comprehensible scientific explanations.