Not exactly true, there are tons of parasites that snake can carry. Most are internal ones. There is however little chance of any external parasites that a rat carries effecting a snake much or at all. Any external parasites ingested would be digested. Ticks can and have transferred from mammals to snakes but are super easy to deal with so I would not risk a regourge for something so minor. Re-gorges can be a serious event in a snake and should not be treated as a minor issue. Parasites are generally less a concern than re-gorge is to my mind anyway.