In 2015, Turnbull published an article ("The Ninja: An Invented Tradition?") that seemed to be a repudiation of his earlier work on ninjas. Here, he argues that the class of ninjas from Igo and Koga as stealthy mercenaries simply didn't exist as a separate category or class, even if stealth tactics were used in war.
I'm not very well-versed in the period in question, but I'm typically quite skeptical about historical texts that make claims that take such a strong stance on popular historical subjects e.g. saying "pirates, ninjas, cowboys, etc. were nothing like you think!" has itself become a brand of pop history wherein denying most of the claims of previous histories sets a historian's work apart from others.
Turnbull's work on the ninjas, and their historicity, has come up here more than once before, While you're waiting for fresh answers to your question, you might like to review this earlier thread, led by u/NientedeNada, which contains links to the most important of these discussions and assessments.
In addition to the thread linked by /u/mikedash, our Sengoku Japan flair /u/ParallelPain has a more detailed deconstruction of the Ninja here at /r/badhistory, and based on another earlier comment by them and also other comments by /u/NientedeNada over the years such as this it appears to be the case that Turnbull's article was not an act of sensationalism in itself but a repudiation of earlier sensationalism.