That might explain the document, but that doesn’t explain some of them keeping slaves themselves. Jefferson even went on to be president and while he did things to restrict the trade, he kept slaves and opposed some measure to restrict slavery (for reasons much as you describe).

https://www.crf-usa.org/images/t2t/pdf/HowShouldWeJudgeOurNa...

https://en.m.wikipedia.org/wiki/Thomas_Jefferson_and_slavery