I can't vouch for the specific number but the IFR with good treatment is very different from the number you get if healthcare is overwhelmed. There's a tipping point where a lot of people that normally would "just" be hospitalized won't make it, and the scary part is that it also affects the 45-60 years old.
You would also have to consider people who cannot be treated for other diseases (e.g. appendicitis), compound effects on the number of available beds due to doctors and nurses being sick, etc.