The idea that characters, figures, and events of myth and folklore are rooted in people and things that occurred way way back in history is called euhemerism, named for the philosopher Euhemerus of 4th century BCE Greece. Is it correct? Well, I'll let others speak to that:

• Were the Greek gods and goddesses real people or is it all just fictional stories passed on over generations? Looking for unpopular opinions on Greek mythology. by /u/itsallfolklore.
• The same topic with discussion about the Greek hero Perseus (though a focus on Arthurian legend), again by itsallfolklore.
• You’re asking about Greek, but for a bonus round, we can take a look at Norse legends in Myths and legends as history tools by /u/sagathain.