I think this is correct:
The inputs were named so front labeled designations for playing or recording tapes from/to Tape DECKS (often two Tape Decks) made sense.
Tape DECKS have internal equalization, then it’s line out, line in. So, you can use any line/tape in/out/loop/ to play any line level signal. Also any Tape DECK into any line input.
IF it says Tape HEAD, then it is only good for a tape signal that has not been equalized. The device would have internal tape equalization, similar to built-in phono equalization.
Therefore, Tape HEAD input cannot be used by any other device, line level or otherwise, just as a MM Phono input which has built-in phono equalization can not be used by any other line level device, or MC weak signal.
Certain era equipment, prior to standardization, had options for various equalization curves for both Tape and LP’s, they had several built in equalization circuits, all fed from the Tape HEAD or Cartridge.