It's not possible for the virus to mutate to a less deadly form?
sure that's possible.
but the doctor has no evidence that has happened, he's just pulling at random hair-brained hypotheses to explain a phenomenon that really needs no explaining (his patients are less sick for the same reason that the average age of coronavirus cases in the us has dropped) despite not having any supporting evidence and tossing those to the popular press who distort the hypotheses even further.
keep in mind that in order for a mutation to have any meaningful effect, it not only has to come into existence, it also has to spread at a faster rate than other virus. this at a time when the incidence of the virus has been dropping rapidly in italy, movement with italy has been controlled and borders have been closed. the incidence of the virus in all neighboring countries has also been rapidly decreasing.
even granting that a beneficial mutation exists in this guy's hospital (no evidence for this exists), and granting that it has spread locally (no evidence for this exists), what mechanism could possibly promote the spread of this strain of virus to other areas where the virus is already widely present and then promote the new strain outcompeting existing strains already present?
it's a farce and the popular press promote it because they're just completely scientifically illiterate.