obviously, the priest who is celebrating will not be hearing confessions during mass, but other priests who are with that parish may.
Sunday obligation MAY be the only time some people can get to Confession. ONe can certainly participate in the Mass while standing in line--and insofar as the Confession will ALSO gain graces, missing the 5 minutes of Mass necessary for the good confession is NOT the same as missing Mass altogether. (That's just the practical aspect.)
Besides, a soul in the state of mortal sin NEEDS forgiveness--precisely what the Pope is driving at with this instruction.