Because his bestest mate heard him say it in a pub, who probably heard it from a friend who heard it from a friend.
So he's not a versatile career criminal who films his crime of rape of an older woman and the Lord alone knows what else ~ has convictions for molesting children ~ is a drug dealer ~ with access to expensive vehicles ~ is a prolific and obviously very good at it burglar ~ whose phone activated antennae in Praia da Luz just before Madeleine disappeared.
But as Amaral confirms, managed to slip through the net because he wasn't in when the police called on him ... I don't think for a minute that his convictions are reliant on a string of friends of a friend. I think he's a nefarious person with a criminal record which proves it ... which of course doesn't make him guilty of being Madeleine's abductor ... but it sure does make him a person of interest worthy of further investigation.
If nothing else is enjoyable about this the twisting and turning of sceptics whose main battle cry was "JUSTICE for MADELEINE" at the same time as they pilloried her parents is certainly giving me time for a wry smile: the Germans are doing their level best to do just that (justice for Madeleine) and are not doing too well in the popularity stakes as a result:(