On top of that, you can't guarantee that young people will only get it. With no travel restrictions, it can easily spread to the elderly.
If we assume everyone in the UK is under 50 and there are 60m people and we want to infect 60% of them that's 36m people infected, and 1% of that is 360,000 dead.
That's ignoring all the people who do have comorbitieis (diabetes, high blood pressure, etc), and all the people over 50.
It's also ignoring all the people who eg get into road traffic accidents and need an ICU bed, which won't be available if we cram hospitals full of covid-19 patients.
https://www.worldometers.info/coronavirus/coronavirus-age-se...
That's not a great way of measuring the death rate during an on-going epidemic. They should be measuring the number of people who died today / number of cases a week ago.
For the modeler, the purpose was estimating the true number of cases (which they estimate as current_deaths * 800 for a first-order approximation) -- but similarly, you could use that to estimate CFR by taking (num_cases / 8) as the denominator. Except, of course, that you can't realistically estimate num_cases. With the exception of Diamond Princess and possibly South Korea, everybody's numbers tell you more about the number of tests they run than the number of infected that exist.
One doctor from John Hopkins, for example, estimated that there are between 50k and 500k cases in the US as of 3 days ago -- and my own admittedly-amateur estimations using CDC numbers for deaths from influenza and all-cause pneumonia could only set an upper bound of ~400k cases as of March 1st
Using the total number of people infected today means they're including a bunch of people who may go on to die, but they're not including those deaths yet. So they're making the death rate look smaller than it is.