Earlier quoted context omitted.
Has this been demonstrated? Is this true? Is this really how the immune system works?
Yes, for instance in this 2015 study on influenza: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4342672/ Higher initial dose results in worse outcomes. Low dose still helps build immunity. Covid19 should be assumed to work like other RTIs. (This is also why obsessively washing your hands and cleaning your house with bleach isn't a good idea, because being in too sterile an environment harms your immune system.)
Hm, okay, at first I just saw the very linear response and wasn't sure how that translates low dose still helps build immunity, but then went back and read your comment up thread, which captures it better: the partial sickness gives partial immunity, thus "priming" the body for the full-blown thing.
Though getting the active transmission rate down could be more important in some places.