It's probably worth noting in general that calling something a "myth" doesn't necessarily equate to believing it never existed at all in any form. It might mean that sometimes, but it can also mean simply that one believes the stories and legends around the person/thing/event in question have been exaggerated and embellished compared to the underlying facts.
You're right. As an example, I sometimes hear people say that Christianity is a myth. Folks who say this are not denying the existence of the religion, or of priests/pastors/deacons, or of the buildings, or the believers who fill those buildings. In those conversations, they clearly believe that those things exist. And yet, they are sincerely calling the religion a myth.
Everyone knows what I mean here when I say this, regardless of their own view of Christianity (or any other religion, for that matter). For those who think that Solo was denying the very existence of a people called Jedi, try looking at what he said from that perspective and see if it doesn't clear it up for you.