what does this mean exactly?
At current rates if the trend holds, it is estimated that hospital systems will be completely overwhelmed. To not overwhelm the system, you need to ensure that
# of people needing hospitalization == # of people hospitals can support. We know the right hand side(because we know how many beds we have). At that level, you can estimate that the COVID fatality rate is exactly as you would expect it to be. Beyond that point, folks needing critical care get locked out of the right amount of medical treatment and the fatality rate starts rising.
Boris Johnson's plan is to allow 60% of the population to get infected in a phased manner. The reason is if 60% get it, and 40% don't, at most, the average transmission should be (40/60) which is no longer exponential and controllable.
The article calculates that if 60% get it over the next 1 year, then as per the above,
# of people needing hospitalization >>>>>> # of people hospitals can support. So, while Boris promises fatality rate to be controlled, in reality you will still see a higher fatality rate. To ensure that the left hand side equals right hand side(and for Boris' plan to work), we can only allow 60% of the people to develop the infection over 10 years and not 1 year. That will support hospital capacity. Which is infeasible.
The solution should therefore be to squash the virus and wait for a vaccine.
(Note that none of this is my inference, I am merely interpreting the guy's article for you)