What have these sins to do with legalism? At first site, nothing, as the Law, the Ten Commandments demands that all avoid these sins and clearly defines what these are.
Christ is reminding the Pharisees and "some of the scribes" what the serious sins are. Washing hands before eating is important, but, hey, this list of horrible sins is much worse.
This is not to say we should not wash our hands, or follow rituals, which is the mistaken impression gleaned from some who read this passage. We do not throw out good and holy rules, but we know what priorities are in the spiritual life.
One must rid one's self from mortal sin first, then venial, then imperfections, in that order.
Christ is taking the Pharisees back to square one, as it were.
It is too easy to keep the little rules and miss the life of constant metanoia, constant repentance and redemption.
Christ let his apostles eat without washing as an object lesson for the Pharisees.
Christ goes farther than the Law at the Last Supper. He is a teacher and gives us clear parables in action.
Washing is outside, repentance is inside.