The article does seem to say the swab was on him. I agree it may be the reporter's incorrect assumption and incorrect reporting. There's a lot of that in this case as there often is in any case. But when there is a named source that is quoted in the article, it does imply that info about what BK had on him per the warrant came from him or the idea was reinforced during the interview.The article is not quoting a named source, as you have indicated, to say the swab was 'on him when he was arrested', instead, it states:
"The warrant shows he had on him at the time: .... ONE CHEEK SWAB"
But the inclusion of the swab on the 'person' receipt is the same as the warrant shows on the receipt from the family home when it says 4 swabs, which we know were not collected from the home, but administered to BK and then taken/collected.
It seems to instead be the reporter's own interpretation, which seems to be different from others, as indicated by the quotes I posted upthread. Common sense seems to prevail, but perhaps this particular reporter was a little deficient in that regard, imho.