The main confusion is the modern attempt to reinterpret scripture in order to bring it in line with the gospel according to Saint Abraham. Onesimus was Philemon’s slave. He had run away and was with Paul. Paul didn’t tell Onesimus “good for you” and he didn’t tell Philemon that he was an evil slaveholder who needed to free Onesimus. Instead Paul told Onesimus to return to Philemon. This makes plenty of modern Christians uncomfortable so we get these convoluted attempts to explain that slavery away as “indentured servitude”.
Paul merely told Philemon to treat the slave as a brother. Many slaves were freed by the Romans. Slaves often lived better then free men in biblical times. Slaves were very common back then, The Romans had many slaves also.