I think the terminology is a bit muddied here because AMD has historically referred to physical cores as "modules" and logical cores as "cores" (although their current spec sheet [1] seems to use "cores" and "threads" in the way that most understand them).
So in a dual-socket setup, 2 x EPYC 9754 would indeed yield 512 threads (logical cores), which are backed by 256 physical cores.
The author pulled it from an article linked in the previous sentence. The numbers don't even add up unless it was a mistake or I'm missing something.