Is it considered defamation to use phrases such as "So-and-so committed that sin" or "One of my friends engaged in a heinous act," even if the person's name is not mentioned?

Phrases that do not refer to a specific person, nor point to a known individual, are not considered to be exposure or backbiting. So, if a person says: "Among people there are those who say such-and-such" or "do such-and-such," there is no harm in that, provided the listener does not understand it to be a specific person. It was part of the Prophet's guidance, peace and blessings be upon him, when he wished to condemn something, to say: "What is wrong with people who do such-and-such?"