To my ears, at least, Timbuk 3 sound too far removed from Roots styles to be called Americana.
Another example: the Stones clearly display Country and Blues influences but still, I wouldn’t call them Americana.
Iron and Wine brings up the sticky nature of defining what Americana is. To me, "Our Endless Numbered days" reminds me mostly of 60’s/70’s singer/songwriters who were influenced more by Folk than Tin Pan Alley. It can seem arbitrary after a certain point but yes, FWIW, I would regard Iron and Wine as Americana.