Does this case constitute plagiarism?
Yes, the use of unattributed material, or use of quotes without indicating they are quotes even with attribution, is plagiarism.
If it's just a few sentences in a long dissertation, it's plausible that some of it was due to lazy note-taking, like copying the words of others into an intermediate document and not realizing months or years later that they were copied. However, laziness is not really a defense. Maybe a mitigating factor.