Rule 7.52 Separate Column Modifiers

[Coniglius]

5.65 P.2 (below) also seems to suggest that a transported unit is attacked via the transporting unit: note that the transport conveys either its advantages or disadvantages.

While personnel units and leaders are normally immune to anti-tank fire, they are eliminated if a transport carrying them is destroyed by anti-tank fire. Conversely, a personnel unit loaded on an APC is protected by the APC's armor value, and is immune to all results on the direct fire and bombardment table except X, 2X and 3X.

Yeah I  came to the same conclusion a little while ago. It's area fire against the entire contents of the hex, not just the wagon or the towed artillery piece.  What happens to the wagon happens to its contents as well.  End result was a morale check, which the wagon failed, so the wagon is disrupted, the artillery is unloaded and sitting on its towed side, and the LT is cowering under the wagon.