Not sure... what is a real 58?
As I'm sure you know, traditionally it's by seat tube. Their 59 is a 56 by that sizing methodology. Also, of frames of similar geo (Venge-like let's say), their 59 conforms to 56 in virtually every other brand. So conforming to what's the norm for that kind of geo in 58 would be my definition, and with the particular geo of this frame that probably means a headtube of around 200, 580 seat tube, reach around 395, stack around 600, and what ever the tt is resultantly.
They've chosen a nominally larger sizing scheme to hide the fact that they don't make any larger frames. But if you're tiny then they're great ... because they have a bunch of REALLY small sizes. Given the way their size distribution is skewed they'd have better chosen HOBBITS instead of ELVES for their brand.