Addendum:

>> Unfortunately this new info says nothing concrete at all about
>> the actual S1 CPU cores or the current state of development...

> "there will be opensource Power10-compatible CPUs available. [...]
> Solid Silicon [...] promises to deliver these new opensource Power10 CPUs."
> https://blog.power-devops.com/p/if-you-are-in-the-us-you-probably

Page 12 of this presentation has some info on the S1 CPU (probably taken from Solid Silicon's TechXchange slides, which I so far haven't found a public link to), which say that it will have 18 or 30 Power10 cores. To me this seems to indicate that there's been no CPU core development at Solid Silicon, which isn't necessarily a bad thing as IBM cores' reliability couldn't be matched anyway. This would leave the on-chip controllers or even just their firmware as points of differentiation from IBM Power10 CPUs (as well as non-technical matters like price, availability etc., of course).