This is what the Forbes article reports the Washington Post saying that Minnesota health officials believe, and it seems to make sense.

I haven’t seen any research on this, but is there a chance being introduced to very small viral loads could actually improve your expected future health? We know that larger loads are harder to fight and produce worse symptoms, but what of “mico-loads”? It would seem they should almost act as a semi-vaccine: you get an easy to fight and low symptom load once, then you’ve created antibodies for the future. So long as you continue to be exposed to micro-loads in day to day life, those antibodies shouldn’t go away, and you’ll be safe from any potential “mega-load”.

I don't know if there's data specific to SARS-CoV-2, but this is the mechanism behind inoculation (as opposed to vaccination). This method was used to reduce smallpox deaths before vaccination had been developed.