In the final episode of Nadia of the Blue Water, Captain Nemo reveals to Gargoyle that there are no more Atlanteans, and that he (Nemo) is the last one. But Nemo has a daughter; Nadia, and he may or may not have made Electra pregnant. So how can he be the last of the Atlanteans?
From what I can tell, you are correct in saying that Nemo is technically not the last of the Atlantians. The most logical reason that I can think of for him saying it is that he was the last person to grow up and experience the Atlantian traditions and society. Because Nadia grew up as an orphan who didn't know her background, she didn't have experience with that society. The society was gone, so Nemo was the last person who would experience that society.
I think Nemo said that because all the people of Tartessos had become humans for centuries (mix). The last "pure" Atlanteans were Elisis and his wife (and thus their two children). So everyone else in the city (including Gargoyle and Electra) were 99% human. Even if he had been raised AS an Atlantean (as he said), Gargoyle couldn't get another DNA than his... He didn't know (who did, except Nemo ???)... Poor one...
"Proof" : Nadia and Jean' son, Nemo and Electra's son.