They seem to be using "The Hobo’s Method" as well.
Do we know who did their branding?
According to Brand New [1], the branding was done in-house by Ian Collins [2].
[1] http://www.underconsideration.com/brandnew/archives/simple_b...
For example on https://www.simple.com/, the logo in the header is simpler that the logo on the credit card.
They also show the 3 versions they use in a blogpost: https://www.simple.com/blog/simple-branding