Per Google “readability” is not a direct ranking factor. But Yoast correctly is trying to make sure your content  “mimics” human speech as much as possible. If something is “hard to read or understand” then it stands to reason that algorithmically, it may struggle to rank. The “readability” recommendations are to help you make content as “simple for users to understand as possible.” As you may have heard me say multiple times: “write for your users and drunk adults.” You do that, and readability is never a concern.