Daas,
“אין צדיק means there isn’t a tzaddik (who doesn’t sin), not that a lower level tzaddik doesn’t sin.”
Of course that is the pashut pshat in the pasuk. The Tanya is certainly not the simple and logic pshat.
Otherwise it wouldn’t be a “Chidush”.
Keep in mind, the definition of Beneni per the Ba’al haTanya is certainly a new created pshat – he is the first one to claim so.