The whole point of drastic social distancing measures is to spread out these inevitable infections over as long a period of time as possible. The same amount of people end up infected but not in such a short period of time that the health service can’t cope so the death rate is much much higher. That’s the whole “flatten the curve” thing you keep hearing about. Surely you understand the concept?!
He is talking about another effect of an epidemic.
Lets describe it this way. A perfect measure would be in his case this scenario:
All under 30 years get infected in the early period and at the same time, when the health system is not overburdened, and in the best case fewer serious cases develop and thus fewer resources of the health system are needed.
And they then can go back into society without spreading it to the more vulnerable. So a substantial part of society does not spread it any longer. The curve flattens and resources can be directed to the vulnerable.
Perfect idea for the UK would be starting this weekend:
All children and youth go to organized 2-3 week youth camps, get infected and quarantined with the neccessary medical resources for serious cases and after three weeks you can send them back to their grandparents without infecting them en masse and thus overwhelming the health system.
Same for the age group up to 30, a 2 week long music festival. They won't even mind the mild sysmptoms, it would be just another massive hangover. And after that they can go back into society and their pubs ;-)