Yes, Kenobi used Ataru
This is an interesting question. In part because it's difficult to read much into a three-strike duel.
To break down the duel, as you mention in the question, Kenobi starts with a Soresu stance and shifts to Ataru (presumably to trick Maul). One of the characteristics of Ataru is broad, sweeping, strikes. And Kenobi's first strike/parry is definitely this as he brings the saber in a full 360 degree rotation to block Maul's strike to his head.
However, Kenobi's second strike is more in keeping with a Soresu defense. You can see that he takes a small hop backwards and brings his saber down to block with the smallest amount of movement necessary. Minimization of motion is one of the characteristics of Soresu, and is in keeping with Kenobi's prefered style.
Finally, the third, and killing blow was another full circle strike from around behind Kenobi's head, through Maul's saber and torso, and following through back to nearly the original position. The sweeping nature of the blow is Ataru, but the choice of action is very much Soresu (capitalizing instantly on mistakes).
We could discuss this duel until the cows come home, talking about motivations and the like, but I think I would break down the fight as follows.
Kenobi opens with Ataru, his stance almost mandates this. Additionally, he chooses to meet the strength of Maul's blow, so for that reason, the sweeping blow was the one he used. His second strike was instinctive, as evidenced by the small hop backwards that is universal for, "I need to move right now without thinking about it", as well as being in the Soresu style. Had he continued focusing in Ataru after Naboo, this instinct would have almost certainly been a flip over Maul's saber. The third and final strike returns to Ataru, his choice for the duel (or at least the beginning), to bring an overhand blow down on Maul.
Ultimately, what we see is a true dueling master. One able to hybridize the forms and adjust as necessary. The second strike betrays Kenobi's love of Soresu, but I think the final answer is that Kenobi chose to open the dual in Ataru, and ended the fight before needing to reevaluate.
To answer the question posed in your comment above: Yes, it is still Ataru without the acrobatics. "Another characteristic of the form was it's fast, powerful strikes from multiple directions", which Kenobi did utilize.