Indeed, if someone was really "prosecuting" by a 5-consecutive-word rule, I think a would-be plagiarist could beat that by going through the text and substituting some pronouns and prepositions, rearranging word order here and there, etc, while still retaining the sense of the original. He'd have to be meticulous to make sure that he made at least one such change every five words, but in principle it would work. But I'd think by any reasonable definition it would still be plagiarism.
To take a trivial example: Winston Churchill famously said, "I have nothing to offer but blood, toil, tears, and sweat." I could write, "I have nothing to give but blood, toil, sweat, and tears." If I actually claimed that sentence to be original, I would surely be guilty of plagiarism. But it has no five consecutive words in common.