Intuitively, immunity debt is using the same logic as vaccines, that the immune system builds defenses to infections when it is exposed to them (and not before.) But I'd never heard of 'immunity debt' as a term before and I am assuming that it's promoted by people that are also anti-mask and anti-vax.

Anecdotally, I played a lot more in the dirt as a young child than any of my children, and I had much more severe asthma and allergies than they have, including the one that contracted RSV as a two-week old infant and went on to get sick just as often as their two siblings.