People, it's not hard to understand what the actual devs are saying.
Their target hardware for their software, which is PC and the lower common denominator of the two base consoles, the 10.3 TFLoPs PS5 as their minspec. This way they can target the enthusiast core audience with a significant installed base and have the best looking/feature complete currentgen game they can possibly make.
Are they saying that BGIII can't run on the S? No. It takes time, months, if not more than a year for a stable optimised code to run sustainably, because MINSPEC MATTERS. Everything from asset creation to level design to npc interactions (and here co-op) need to be worked around and scaled down ( or in the internet's favorite hyperbolic word "downgraded") to run on Series S. But they're not rewriting code.
Think how The Witcher 3 and now the Arkham Knight are running on the Switch vs the definitive way to play them - the PC > PS4/XB1 > Switch. I know it's a slightly different comparison because of different architectures (x86 vs ARM) and outsourced developers, but the context here is the same.