I think people are reading Thanos' influence fromhis line "Fine. I'll do it myself". This implies that he was behind the events of the film and it didn't go his way so now he is stepping up personally. I still think his role is not certain though.
Great pointThat scene seemed pretty ambiguous. It was almost like they didn't know what to do so there were like "Okay, Thanos again!" Ultron wasn't trying to retrieve a stone for Thanos, and as far as we know Thanos doesn't really care about Earth or its destruction (he offered Earth to Loki). That scene makes more contextual sense to be set after Ronan failed/betrayed Thanos.