I consider it a typo; the author clearly meant the EU, but wrote Eurozone for some reason. I can't entirely blame them; the many European treaties are a complicated mess.
Unfortunately as a citizen of one of the founding members of the EU, I don't even recognise half the flags in the diagram.
Give me the names and i'm sure i would know them, ask me to point them out on a blank map and we're straying back in to embarrassing territory again.
Off the top of my head Russia, Ukraine, Greece, Moldova, Turkey, Georgia, Northern Macedonia, Albania, Bosnia & Herzegovina, Serbia, Belarus, Romania and Bulgaria are all there anyway.
I think I've still missed a handful but I'm pretty happy with my European flag knowledge apparently from that.
Acknowledgement: I would like to thank the Eurovision Song Competition apparently for my education