I skimmed through the affidavit & couldn't find that claim. It was difficult to read the content so I didn't look to deeply into it. Could you post a quote of the claim if it's there?
To support the idea that it was he who engaged in these messages, they claim that you can only be logged in to kik on one device at a time and have evidence that he was using the device for other purposes around the same time and in the same location as kik for at least one of these conversations.
That's my reading, anyways. Not a lawyer and have trouble with the stilted language in these kinds of things.
(As opposed to testimony, where you have someone else testifying that they SAW him doing it, but no way of proving that post facto)