I am not a lawyer, but in general words are copyrighted; ideas are not. The context doesn't really matter too much, if the words are unique and convey the same idea for a different topic (which these almost definitely do), then you do not have the right to copy them! There is no excuse like "I was talking about horse breeding, and they were talking about aircraft mechanics."
A valid excuse is that it is a common formulation, but you'd have to prove that by finding other (preferably earlier) sources using the same words without any attribution. If it is a common formulation, you should be able to do that.
Ultimately human judgment is used to decide whether you plagiarized something or not. A jury, professors, or peer reviewers, or editors or publishers. If grammarly can find it, they can find it, because none of those (or lawyers) they aren't above using grammarly or similar software as a first check.
I'd say if grammarly flags it, and you can't find any other sources yourself, bit the bullet and realize somebody beat you to the punch, but it means you produced publishable writing, so there is that. You can check with a lawyer to see if it is "fair use", but it probably isn't without an accompanying citation to the original.
Bite the bullet and be an author, use your imagination to find a way to convey the same sentiment without plagiarizing somebody else.