Though the wording of the article is unclear, the reference to Sextus Empiricus is only to the question of how many schools the "Academy" has ramified into.
This a direct (albeit somewhat loosely translated) quote from Cicero's Tusculanae Disputationes I.17:
Geram tibi morem et ea quae vis, ut potero, explicabo, nec tamen quasi Pythius Apollo, certa ut sint et fixa, quae dixero, sed ut homunculus unus e multis probabilia coniectura sequens. ultra enim quo progrediar, quam ut veri similia videam, non habeo; certa dicent i, qui et percipi ea posse dicunt et se sapientis esse profitentur.
My quick literal translation of the whole passage:
I will gratify you and explain, as I am able, what you want [me to explain], yet [I will do so] not like Pythia, so that the things I say are certain and stable, but as one little man out of many seeking what is more probable through conjecture. For I have no further place to go to than that I see what is like the truth. They who both say that they can perceive what is certain and profess themselves to be wise will say what is certain.
- I think coniectura is ablative here, so I've translated as "through conjecture."
- It might be possible to construe e multis differently: "seeking what is probable out of many [other things]." The position after unus made me favor the other interpretation.
- It's a bit hard to translate the last sentence literally without making the word order awkward in English, so I opted to repeat "what is certain" and change the word order.