the paper admits that it is likely that people's behavior would change
but shows the numbers for how the outbreak would affect the whole population absent any intervention. If you don't think that number is relevant, fine. But the results also showed that if we take just a few basic mitigations - that some people think are all that is necessary - we'd only drop the number of deaths by around 50%. Larger measures like social distancing of the full population are what you'd need to do to get to the much lower number of deaths.