The boyfriend was swabbed on May 3 - the day AFTER the primary. I think Nifong feared the press learning before the primary that non-player semen was recovered by the SANE, and the focus becoming Mister X instead of soon to be indicted Seligmann and Finnerty.
I think Cheshire would have been trumpeting the fact that Nifong knew for a long time that CGM had sex around the time of the incident, and that Nifong withheld the info from the first DNA report.