There are two parts to this:
1) Get the RNA into the cell in the first place. Anything you want to get into a cell has to pass through the cell membrane, and if the molecule is any larger than H2O, the only way to do that is with a transport mechanism you would find within the cell membrane. In this case, the transferrin receptor that transports Fe from the bloodstream to the interior of the cell.
2) Cause the transcription interference in the DNA itself, as described by the GP. At this point, the transferrin receptor is no longer at issue. While normal cells will definitely uptake the RNA, the idea is that normal cells won't be affected by it because it is designed to interfere only with RRM2, a cancerous mutation.