I can't really interpret 100% the first message. It looks like they are telling you where in Chalkidiki is NOT allowed to dive (Porto Koufo and Toroni), suggesting that everywhere else it is allowed - unfortunately I am not familiar with that area. They also seem to suggest that you should dive with a local operator (school as they call it) - which is definetly NOT true. Certified divers can dive without a local operator here in Greece (excluding forbiden areas of course).
The second image/message is the list of officially registered organization. Apparently the only officially recognized/registered organizations, so far, here in Greece are : ANTI. PADI, the Greek one (EOYwhatever), SSI and IAN(T). According to this message, any other organization is excluded i.e. divers holding certificates from unregistered organizations are not allowed to dive in Greece!!!
Keep in mind that people who have any certificate (eg OW, AOW) with any of the registered organizations will be good to go.
If I was certified with any other organization (eg CMAS, GUE etc) I would send them a query/question about it. For example., I've seen online GUE instructors advertising their courses locally - maybe they operate under the radar? Who knows? In anyway the local market is pretty small, so I'd guess "smaller" organizations don't bother registering.
I am "only" PADI, SSI and IANTD so I am covered - no point for me to ask around for other organizations.