I take certainty to be a special case of uncertainty. Regarding proof, the relevant bit is here:
This argument does not apply when the agent is unaware of the structure of its decision tree, so I provide some formal results for these cases which bound the extent to which preferences can de facto be completed. … These results apply naturally to cases in which agents are unaware of the state space, but readers sceptical of the earlier conceptual argument can re-purpose them to make analogous claims in standard cases of certainty and uncertainty.
I take certainty to be a special case of uncertainty. Regarding proof, the relevant bit is here: