That's not mentioned by the article, yet it seems important.
4 ounces is 110 ml - a reasonably bug gulp. It's not the kind of dose that you'd get from accidentally licking your finger, or absent mindedly eating an apple that had rested in a splash of cyanide.
The article is right about modern inquests needing clear evidence before they record a verdict of suicide. But that was even more so in the past, when suicide was a crime, and much more likely to be seen as sinful.
Sorry, I couldn't resist. I understand why suicide is illegal (so the cops can intervene before you kill yourself), but I still find it amusing to think about punishing people for killing themselves.
That is dumbfounding. It alone calls the verdict into question.
Edit: according to http://sciencemuseumdiscovery.com/blogs/insight/the-spirit-o..., autopsy found that Turing's stomach contained four ounces of cyanide solution and the apple had nothing to do with it.