Antenna conductors are the wire that the antenna is made from, note that the phrase is under 'Size of wire-strung antenna'. There are many configurations of wire antennas such as a dipole, a loop or a rhombic.
Lead-in conductors are the conductors that go from the feed point of the antenna to the radio system. They can be coax or balanced feed line, like twin-lead, ladder line or window line.
As you can see, the antenna, a folded dipole, is made from wire. The part labeled 'antenna feeder cable' is what the NEC refers to as the lead in conductor.