Well, that settled it. A Germany based colleague Andreas Schilling is rather confident he's not under any embargo, so he posted two photos on Twitter of packaging showing some specs and yes, the Radeon RX 590 naming.
By posting the two photos it is now confirmed that the Radeon RX 590 is a real thing, and you can read that it is a FinFET 12 aka 12nm fabricated product. The earlier leaked result has demonstrated the product (RX 590) to be clocked at ~1545 MHz clock. The suggested retail price would hover in the $299,- which is a good chunk more expensive than a RX 580 8GB at $229.
The respin GPU would be the 3rd iteration of Polaris, now noted as Polaris 30.
German Editor Confirms Radeon RX 590 as 12nm Polaris 30
