I think part of the reason the songs sound as good as they do is that both of Eden and John have been passionate about this music for a long time. They have pictures on their web site of them standing in front of shelves filled with old 78 records that they've collected over the years. They appear to spend a great deal of their time getting together with like minded musicians and playing this music just for the love of playing it. That love shines through in every song sung and every chord plucked and played. When you hear people that excited by what they are doing you can't help but get caught up in it.
You can also tell this just isn't a fad for these two. They'd be playing this music even if there wasn't the renewed interest we've been seeing over the last few years for more traditional forms of music. When Eden and John play a song like Little Hat Jones' "Bye Bye Baby Blues" the song sounds like it was written for them. They may not have the most polished of voices or be the slickest of players, but this music wasn't written by or performed by people who were either. I think if you're going to sing "Do Dirty Blues" by Bertha "Chippie" Hill you have to be a little rough around the edges.
If you go to Eden and John's web site you'll find links to all sorts of interesting information about the music they play and where they found it, and places you can see them, and others like them, playing. For those of you who like beautiful old guitars, even if only to look at them, they also provide a link to a site that sells restored guitars from the 1930s, featuring some very rare items made by Stella. I mention this because I think it will help you understand how they are able to bring music that's nearly eighty years old, if not older, alive without sounding affected. They have taken the time to understand the music and dedicated themselves to it until, and you can tell this by listening to them, they live and breathe it.