I don't think essence here is Platonic. It's CREATIVE. In other words, what's being said here is that first we have our material / social conditions (existence), and then we develop our characters and choices over those conditions as we gain responsibility for creating ourselves and our lives (essence, which is our creation to a great extent).
Hiram Yes. Essence here in the core statement (Existence precedes essence) is not Platonic since the word has already been demoted from being the subject to becoming the object in the sentence. It is superfluous to bother ourselves extracting the metaphysical essence of "essence." The statement only asserts the primacy of existence over essence. Therefore, the Platonic concept of essence has been broken here.