It's only a problem if you have a lazy assessor who just takes the similarly score and runs with it.

When I was marking essays 10 years ago, I saw Turnitin as a manual review tool. You feed an essay into the program, and it would give a percentage for similarly. It would panic students to see that the program was finding some similarly, but unless it was very high, the score was mostly useless.

The useful part is where it highlights the passages that it thought were problematic. Almost all the time those highlighted passages were properly attributed sources, so not plagiarism. If you had an excessive amount of cited material, I might give a lower mark because of less original thought, but it's still not plagiarism.