And since no one believes it anyway, why would he tell it to the Longbottoms, who certainly won't believe it, unless it were true?

A good question to ask. Might he not be telling it for the benefit of Harrymort? I still don't understand why he would do this, but at least Malfoy has a track record of saying confusing things to Harry. It could mean "I am no longer your follower" or "You need my support" or "I am the one in charge of your Death Eaters now, and don't forget it" or even just "In case you don't know, this is my official position at the moment."