"Mitigation", according to Merriam-Webster, means..".the process or result of making something LESS severe, dangerous, painful, harsh, or damaging" (my emphasis).
A properly fit N95, or KN95 mask can be VERY effective...as evidenced by the fact that medical personnel in highly infectious environments are able to conduct their work all day, every day for years without becoming infected (being vaccinated improves those odds a great deal)...however...as verified and supported by literally thousands of studies, even less than perfectly fit masks of this type will provide a very high level of MITIGATION...i.e. "LESSENING" of COVID-19 transmission...to the wearer...and to others...in fact a study of 350,000 participants by Stanford and Yale researchers...using "Surgical Masks", not KN95/N95 masks...that were certainly not "perfectly fitted"...found significant reductions in transmissibility.
It is absolutely wrong to assert that wearing a KN95/N95 mask has ZERO effectiveness...again, referring to Baron's "MIT Report" on the use of masks, social distancing, etc. in schools...kids wearing KN95/N95 masks were predicted to be able to remain in a class for days with such masks without incurring a "VIRAL LOAD" of COVID-19 "virions" that would cause them to be infected...and enough hours to cover a school day with 'Surgical Masks', which would IMO certainly qualify as poorly fitting KN95 masks.
Also, because every reasonable person understands that few things in life are "PERFECT", we also acknowledge the possibility of infection in spite of mask wearing, so we employ rapid "Testing" of school children, so they can be quarantined quickly, thus allowing their classmates to continue in-person learning...this was also mentioned in my OP as part of the "MITIGATION" strategy.
MITIGATION means reducing the transmission of COVID, so that FEWER people are infected...become seriously ill and flooding our vital hospitals..or dying...it's not that hard to understand.
Link: https://www.merriam-webster.com/dictionary/mitigation