Horace and the Dark Mark

in HBP, once Dumbledore has determined that no one had been harmed/killed and finding Horace transfigured into an armchair, Dumbledore mentions the Death Eaters would have left the Dark Mark had they been at the house, to which Horace respons:

"I knew I was forgetting something! Wouldn't have had time to conjure it anyway."

But in GoF, Mr.Weasley says that only Death Eaters know how to conjure the Dark Mark, so how would Horace manage this feet unless he was a Death Eater? or was this just a flippant comment that Horace makes?