Grammarly's plagiarism check isn't catching copied text

Not you. Plagiarism checkers only find what's in the index they compare against, and newer or smaller blog posts are often not in it yet. A competitor's page from a few months ago can easily come back clean. For vetting writers we use Copyscape, it's built for exactly this (checking web content against the live web) and it's cheap per check. Quick manual test as well: take an odd-sounding sentence from the draft, put it in Google in quotes and see what comes up. And if the writing style suddenly changes halfway through a piece, that's worth a question too, no tool needed.