But I am very interested to hear other interpretations, and the sentence is very open to it. — Mind Dough

It is impossible to be alone and nothing does not exist. Thus they are not.

It's a translation, so absent the Greek and a competent parsing thereof, it's a guess. The significance of the fact of translation should not be overlooked. For example, most of the New Testament is written in the Greek of the time, koine Greek, the more-or-less common language of the Mediterranean for about a thousand years. You would think that translating the Bible would be settled business, and free from error much less controversy. Not so, and some of the problems matter!

In short, you can strain your brain with alternate readings of translations of Parmenides and miss the mark altogether because of the translation.

Still though, it's a challenge. I read it (the translation) as, Being and only being truly is, [and] (the) nothing is not completely and entirely a nothing. In Heideggers' phrase, "Das Nichts nichtet." The nothing "noths."

Heidegger has a lot to say about being, much from the Greeks. Not simple subject, and sensitive to translation, meaning it's good to have a guide through the material.

Heidegger, The End of Philosophy, ISBN-13: 978-0226323831. The title means something other than what it appears to mean.

Being alone is and nothing is altogether not.

It is his way of clearing the decks, similar to how Descartes clears what is from what is not. What is not can't be thought, but what is can be thought, so then being or cogito.