There’s definitely something to learn from the setting of the position. I actually took it from Strategic Chess Exercises, just taking one of the variations of one of the problems. There’s picking a position that it makes sense to debate over, but also a meta thing that you have raised, which I didn’t consider.
…Qc5 is the stronger move, but …Qxb5 still leaves black better off than white. It would probably have been better to have a greater discrepancy in the evaluation of the moves.
The mistake in your reasoning is that after …a4, d6 is not threatening, black can respond …Rac8. As I said in another comment, however, I would expect white to hold the draw in this position, where as after …Qc5, black has a decent advantage.
This is my belief, and why I do not think AI debate is a good safety technique. Once the ability difference is too great, the ‘human’ can only follow general principles, which is insufficient for a real-life complicated situation. Both sides can easily make appeals to general rules, but it is the nuances of the position that determine the correct path, which the human cannot distinguish.