I don't know if we can know how close our understanding is to the original stoics. I know that there are a number of abuses and misuses of stoicism floating around like manosphere stoicism and tech bro stoicism.
I've read more than one but less than all of Holiday's books on it as well as some translations of the source material and I'm confident in saying Ryan knows his stuff and has probably captured the original intent better than any other author I know of.