It’s not illegal to misattribute small portions of a sentence either. There is literally no law against it. We harshly dissuade it at the grade school, high school, and undergraduate levels (meaning, we will give you a bad grade) because we are trying to teach young students not to misuse even a fragment of text with extremely harsh punishments outside of the legal system. We don’t apply the same level of punishment in real life and certainly not through the legal system because it’s not that serious to borrow a few words in the acknowledgement of a thesis. It’s just embarrassing and bad, as long as the concepts are original.

ETA: I say this as someone who has had both scientific contributions and entire introductory sections copied verbatim into other paper. That’s plagiarism. A meaningless sentence copied from my work has as much relationship to serious plagiarism as a fart in a car has to a Sarin gas attack.