Without knowing the inner workings of UEFA it's difficult to say. Gill is UEFA Treasurer so may routinely have access to this sort of information or may have got it from someone involved in the investigation.
At the end of the day, Leterme isn't claiming that the leaks aren't coming from UEFA but only that they're not coming from the CFCB. As I've said, it's a careful choice of words.
But their own procedures dictate that the ongoing investigation will be confined to the CFCB and that includes UEFA, so the fact its probably Gill running to his bum chum at the NYT doesn't explain the initial leak from the CFCB to the wider UEFA community