De la Rionda asked Dee Dee if she and Trayvon were dating (2:26). Her response was an unintelligible grunt, but de la Rionda seems to have taken it as a negative.
Given BDLR's additional "I don't mean to get personal
, but, I'm not gonna ask you any more, other than that," I think he took her going non-verbal at that point as a sign of embarrassment, not as a denial that they were involved romantically.
Hence the brief gentle fishing expedition that got her to admit to "gettin' there."
If he took it as a negative, he could've asked her to state clearly that they were not romantically involved in any way, and pressed further about the nature of their relationship.
(And yes, I know we're talking about BDLR
here, but he did press her for a straight answer on a couple of other points.)