Don't know if Thai police take DNA when they arrest someone on a felony charge, but if so, and if they put it in a worldwide data base, then perhaps it popped up as a match to the DNA from Jonbenet's panties and from under her nails. Or, the FBI could have followed him around over in Thailand until he threw away a coffee cup, taken the DNA from that, and found a match with the DNA on Jonbenet. ABC said it matched.
It has not been announced whether they have a match or not.