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They weren't his solution, no. They were the solution settled on by TfL which he initially tried to oppose. He was eventually convinced of it, but he had already been so adamant about Routemasters (the famous 'ghastly, dehumanised' quote) that he couldn't communicate why he changed his policy. Johnson reheated the policy partly because he thought there would be mileage in attacking Livingstone over this.
Livingstone had the Routemaster policy long before articulated buses came into widespread use. He was convinced by TfL buses that articulated buses were the future. Of course, this about-turn was widely seen as a betrayal and no doubt dented his popularity considerably.