Grammarly is not paying for this. Grammarly is not involved or even concerned in any way. This is not even a "hack" but rather trying to use Grammarly for something it was not designed for, and showing it does not work for that. "Ok, duh, and thank you for your interest in Grammarly..."

The plagiarism check is for self-checking, as in users making sure they did not miss any quotations and references in their work, and thus it does not even attempt to defeat any plagiarism enforcement counter-measures. It's not an enforcement tool.