Welcome Blossom.
Dr Locke performed the DNA test on Rob. The scriptwriters (well one, presumably speaking for all of them!) has assured listeners, via Twitter, that there was no skullduggery with regard to the test.
(Perhaps the SWs regret not being clear enough in their writing of the relevant episodes as they're now having to explain them outside the broadcast.)
It may be that they put those deeply unprofessional words into Doc Locke's mouth (saying he thought he'd met Rob somewhere,) purely to remind listeners that the two had indeed met first in a different context.