Prime Directive/Star Fleet Battles might be some bizarre divergent evolution of Star Trek, largely unrecognizable to modern eyes,
The core of the Star Fleet Battles universe is still basically the original Star Trek. Federation, Romulans, Klingons, Tholians, Orion Pirates.
Then some stuff was from the animated series. Like the Kzin. (They really only come into play vs. Klingons, since they are the ones that generally always fight with and are allied with the Federation, but mostly an alliance of convience).
And then some stuff was extrapolated, like the Gorn becoming friendly with the Federation (since when Kirk didn't kill the Gorn captain, they seemingly realized the whole thing was a misunderstanding). And they extrapolated the Lyrans from the Kzin, and the Wyn from the Orions, just to give those races rivals. And since the Klingons were a conquered race, then surely they must have some subject peoples.
All of the extrapolated stuff makes sense, I think.
I'm not sure where the Hydrans came from. But they are pretty cool. (Still, as they are on the other side of the Klingons from the Federation's territory, they can be ignored in play)
You can pretty much ignore the other stuff, like the ISC and the Andromedans. But the latter are pretty cool, I think. (They inspired the Antarans in Moo 2).
I was actually into the wargame when I was a kid in the 70s and early 80s. It was pretty cool back then, because the universe was sort of "living". But when ST:TNG came out, I think they pretty much pulled the plug on the evolution of the timeline. (Since they didn't have the license for it).
Anyway, for the most part, I think SFB is truer to the original series than the later TVs, or even some of the original cast movies.
Klingons, for instance. Went from being sort of a Mongol analog to samurai with bumps on their head (It's probably the Romulans that are more like the Soviets, I think, though sort of a Roman-Soviet crossbred). There were sentient robots all over the place in the original series, then in TNG, woops, only one. Andorians and other Federation races seemed to have disappeared until Enterprise. Really, even the starship battles became much more faster paced. In TOS and up to Wrath of Kahn, the battles were very slow and deliberate, like submarine warfare.