She says she was the Amazons' chosen assassin "until" they abandoned their great mission--and yes, that was a long time ago. Why only "until"? Evidently, once the Amazons abandoned the mission and started hiding from the world, they had little no or need for an assassin, and Myrina wouldn't have been allowed to spend a lot of time in an Athenian "mission temple" protecting the world.
But, a long time after that, Myrina took up the mission again, solo, and left the island; she says she has been protecting the mission temple (or the modern world of glass and steel --which, by the way, wouldn't even have existed hundreds of years ago) "for decades." One doesn't say one
"has been [doing something] for decades" if one has been doing it for hundreds of years, or if one did it hundred of years ago and then stopped. So her modern, solo version of the Amazons' mission is distinct from the original, ancient version, in which she had been the Amazons' chosen assasin.
If she began her modern mission enough "decades" ago, could she have been, in some ways, the equivalent of WWII Wonder Woman (except that she was an anti-hero)? Sure. But if so, it sounds like Diana may have been born earlier than we thought, since she was born on the same day Myrina left the island and embarked on this modern mission. Another possibility is this Red Panzer (like the second and third versions in old continuity) is/was a neo-nazi operating in more recent decades.
ETA--Here's something I think would work: time passes slowly, and maybe erratically, on Paradise Island relative to time in the rest of the world. So, Diana was born on the same night as Myrina left, but, during the 18 years or so that Diana spent growing up on the island, Myrina experienced at least 75 years of world history. This could help explain several things, including the Amazons' apparent agelessness, why that ageless ness (acconding to Soule's SM/WW depends on their not living away from the island, and their slow technological and social development. It would also mean there have been fewer "raids" than we thought, if every 30 years by the Amazons' reckoning is more like every 90 years by the reckoning of the world. I have no idea whether this is what Johns has in mind, but I think I want it to be.