It's a bit of a contradiction to tacitly support something but not advocate it.
Either you don't advocate something, or you tacitly support it. Can't be both.
Matter of perspective and nuance to the words I used...
They don't believe (their perspective) the flag represents support of slavery ie not advocates (being active and public); however most other people in the US do believe this and the facts back them up so confederate flag wavers tacitly (passive and implied) support slavery from others perspective