No, a cannibal should not interpret Mark 7:19 as giving them permission to eat whatever they want. The things that count as food vary from culture to culture, such as US citizens will often consider the flesh of animals like cows, chickens, and pigs to be food, but won't consider the flesh of humans or animals like horses, dogs, cats, rats, bats, vultures, or human flesh to be food, so if someone were listening to a conversation between US citizens about eating meat, then they should not insert what they consider to be food, but could consider them to be speaking about eating the flesh of animals that they consider to food. There is already a sense that certain animals are mot to be eaten as food, so the issue is which column pork belongs under. So when we have a conversation between one Jew and a group of other Jews about eating food, then we should not insert the things that we consider to be food, but should consider them to be speaking about what they considered to be food, namely the things that God said is food in Leviticus 11 and Deuteronomy 14. Moreover, they were not discussing what type of things are acceptable to eat as food, but about whether it is acceptable to eat bread with unwashed hands. In Matthew 15:20, Jesus was still speaking against being defiled by eating with unwashed hands, so thinking that he was setting aside God's commands against eating unclean animals is a complete misreading of the text, especially when he just spent Mark 7:1-13 criticizing the Pharisees as being hypocrites for setting aside the commands of God.
Jesus lived in obedience to God's law, which included refraining from eating unclean animals, so that is also the way that we live when he is living in us.
