This AI definition is in accordance with how I see it but others may have different opinions.
Americana is a contemporary music genre that incorporates elements of various American roots music styles, including:
Country, Roots-rock, Folk, Bluegrass, R&B, and Blues.
Americana music is often acoustic, but it can also use a full electric band. It’s characterized by a distinctive roots-oriented sound that’s different from the pure forms of the genres it draws from.
EXCEPT that I don’t see it as strictly contemporary.
I’m not familiar with Old 57’s but I’d say T. Joe White is Americana. He’s got Blues, R&B and maybe Cajun influences but his music doesn’t sound like "the pure forms [he] draws from".