What you're saying here is not quite right. As I've written on this sub before, we don't actually know what killed Alexander the Great, nor will we ever be sure. Every now and then someone launches a new theory to fit the (late and contradictory) evidence. None of them are likely to become the accepted cause of death.

Meanwhile, I also don't think we actually have any evidence of the use of opium in Alexander's army. There's very little evidence that the Greeks and Macedonians were aware of any narcotics besides alcohol.

The claim that it was used "as treatment for PTSD" is especially dubious, since we are certain that the Greeks did not recognise anything like PTSD as it is defined today. In other words, the claim that Alexander's army was using opium to treat PTSD is unhistorical three times over: we cannot prove that Alexander's men suffered from PTSD, we cannot prove that they were getting treatment for it, and we cannot prove that the treatment consisted of opium.

That said, if the men of the Macedonian army had adopted opium as a drug, and if this usage had spread to Alexander (though no source suggests this, and all sources emphasize his heavy drinking instead), then sure, it is possible that this was a contributing factor in his death.