As with all good internet journalism, the answer is at the end:

> The immune system is 'not a muscle.' All this begs the question: Will kids have more severe infections after the COVID-19 pandemic if their immune systems missed out "training" during winter lockdowns? Carsten Watzl, the secretary general of the German Society for Immunology, says we may see more infections, but not necessarily more severe ones.

TL;DR: Eating dirt won't work out your immune system because it doesn't work like that