Those RX cards are just the desktop/desktop replacement chips. Those won't be put into an iMac, they have too high of a TDP.
I think your understanding of AMD's product line is far too rigid.
As mentioned, Vega is not just two chips. It's an entire generation, but it just so happens, not all of the versions of Vega are out yet. Vega won't just get different clock speeds either.
In terms of RX, Apple is
already using RX 500 series, albeit with their own configurations binned with power characteristics appropriate for the iMac, and then rebranded as Radeon Pro. I should also point out that AMD actually advertises the RX chips for mobile use. And RX 500X is mainly just a rebrand of RX 500, which Apple is already using. If Apple were to use Polaris RX in 2018, then it'd probably just be named Radeon Pro 500X series.
Or else they could release later on with Vega, just not the same Vega that is in the iMac Pro.