He means that ASCII table has an actual character that represents end-of-text and that terminals respond to with SIGINT, and it is ETX character, 0x03.
The Control modifier on your keyboard basically clears the top three bits of whatever ASCII character you type, leaving the bottom five and mapping it to the 0…31 range. Ctrl+C sends 0x03, which is exactly ETX.
Look at the ASCII table for a clue, this is why it was a convenient thing to do.