as I understand it, it’s not the lack of access that explains the fact that Randolph didn’t father Sally Hemmings children. it’s the main argument justifying the claim that he didn’t. The DNA evidence cannot distinguish between Randolph and Thomas, so the lack of access is the only thing that implicates Thomas and exonerates Randolph. it is possible that Randolph impregnated Sally during his visits to Montecello, particularly if he regularly visited the Montecello slave quarters as you say. But it’s far more likely that Thomas was the culprit.