Plagiarism detection is a somewhat different problem, in that it's looking for specific common text rather than just stylistic choices. Usually it's just looking for high percentages of overlapping ngrams between a test document and documents in a corpus, but two different documents written by the same person wouldn't test positive.
There's an interesting research paper about their algorithms here: https://www.cs.auckland.ac.nz/courses/compsci725s2c/archive/...
And if you search for "Turnitin Plagiarism Algorithm" I'm sure you'll find a few more resources.