I was about to ask what possible conspiracy theory could people come up with to doubt the story of: Guilty verdict, drink liquid from small bottle, die of poison...
But then I remembered that conspiracy theorists are often inspired by a muse that other people don't have access to. So best to just shake my head and find something else to read :-)
Nitpick: the video shows him drinking a small bottle of liquid.
https://web.archive.org/web/20171203203439/https://www.om.nl... says he had potassium cyanide in his blood, but doesn’t go as far as saying that’s what killed him.
https://web.archive.org/web/20171201031537/http://time.com/5... says the container contained a deadly chemical, but doesn’t say it was potassium cyanide.
The UN only looked at whether the court should have prevented this from happening and concluded that, within the rules, it couldn’t and, hence shouldn’t (https://www.icty.org/en/press/statement-on-the-independent-r...)
I couldn’t find the final reports of the Dutch Justice department.
That gives plenty of room for conspiracy theories.