I admit I have not read the article yet but I have done independent research into these stats previously a few weeks ago. I found the 30m quoted number is the monthly cull amount. I have seen many references to the 115m or similar numbers but have no idea over what time period. I had my own ~120m number come up from my calculations: The industry claims it takes 4-6 months to clean, disinfect, hatch and re-grow, and get the chickens to egg-laying age. If you take the ~30m and ~4-6m period, you end up around ~120-180m "missing" egg-laying chickens in the flock at any time assuming this replacement is taking place at expected levels.
where do chickens come from? eggs! If one needs more chickens, one could let the eggs be fertilized and then incubate those eggs. all you need is a rooster in good health. Eggs that are incubated in this way won't be in the grocery store in cartons, because they will be chickens instead.
It's a very different supply chain. You don't just chuck a rooster in with your layers because then you have to inspect all your eggs.
New layers are bred away from the main flocks in relatively tiny volumes, under 1% of layers' eggs. Culls may drive a need for more layers to be raised, increasing that percentage but the cycle rate for these hens in intensive farms is pretty regular already (<18 months) so I doubt they'd invest in the additional hatchery facilities for a temporary population lull.