[Rules] Bombardment Chart - Example

[triangular_cube]

How does 7.5 apply here? It clearly says to apply the die roll to the different columns:
 7.52 Separate Column Modifers in One Attack - In some cases, a column modifier will apply to some units in a target hex but not others. In such a case, make one dice roll for the attack, but use different columns to determine the combat results on the different target units.

I would play it where the Art and Inf each take a step loss and the infantry then must pass a M2 moral check (per 7.6)

That would be conducting multiple attacks though, which is why the rules header in 7.52 specifies that it is still one attack. The interaction, albeit not this specific interaction is included in the example in the rules text. For those that dont have it up, a single split column attack results in an M1 to an Infantry unit and an X to a weapon unit per die roll.

If it was played as you are suggesting, with it being treated as two attacks with cumulative effects then the Infantry would receive an M1 and the weapon would receive an X, then the INF would receive an M2 from the cumulative effect of the X.

But that's not what happens, as the example illustrates. It is ONE attack you are just finding the result on separate columns, hence the weapon receives X and is eliminated, and the INF receives and M2 via the weapon X. The M1 is dropped, again ONE attack, not cumulative. 

When ONE attack achieves an X, one step is lost, not one per unit...

Sorry, I don't understand how this amount to Two attacks. It's one target hex, in one action segment, with two units with the misfortune of being on the receiving end of the combat result.

Two separate, cumulative results = two attacks

It is one attack with different results based on the type of targets in the hex. It just happens to be a disaster for the receiving end. In other cases a very strong attack does nothing, so it evens out over the course of a battle. You can always disregard the rule when playing, but it was clearly written to address this specific example and is clear about how to handle them. The rule clearly says "one attack" and to apply results on two different applicable columns. I was simply answering the initial question which seemed to ask how to apply the rules, you can house rule anything you want to.

The example in the text refutes your methodology. No house rules here.