to be fair they don't promise security, only privacy. at least in their foreword on their website here.
I don't think it's by accident that they don't even use the word secure, or security, on the whole page.
https://e.foundation/e-os/
I've seen claims before where they claim it's better than GrapheneOS. But in what regard? Maybe degoogling and having alternatives pre-installed? GrapheneOS is probably more involved to get the same apps. That's the only way /e/ is better in my opinion