Coupd anyone explain how this works, are we any closer to the dream of being able to boot generic Ubuntu image on a random ARM device like you can with x86?

It's a standard specification for ARM server hardware / memory maps / firmware / etc, so operating systems should be interchangeable on it. Doesn't have a specifically consumer orientated equivalent though.

Its a bit like the way you need more than an x86 CPU, you need an "IBM PC Compatible" system[1] with various buses and peripherals in the right memory locations, BIOS/UEFI firmware etc. for generic PC OSes to work. SBSA standardises something like an equivalent of an "IBM PC" for ARM Servers.

[1] Or whatever you would call it nowadays, as legacy BIOS booting is fading away modern systems might not count as that.

We aren't any closer, we are already there. Generic Ubuntu image already boots on all ARM SBSA devices, just like x86 PC. In fact, booting generic image on any device is explicitly a goal of SBSA standard.