(There’s a funny aside on Thermopylae, and the limits of ‘excellent leadership,’ yes they did well but they ultimately lost. To which I would respond, they only ultimately lost because they got outflanked, but also in this case ‘good leadership’ involves a much bigger edge. A better example is, classically, Cortes, who they mention later. Who had to fight off another Spanish force and then still won. But hey.)
I know this is an aside to your aside, but as an avid Sparta-hater, I want to point out that we don’t have much evidence that Spartans are good at military leadership, and indeed plenty of evidence in the other direction:
Sparta’s opponents often won using clever, innovative tactics, such as at the Battle of Phyle (404 BC), the Battle of Olpae (426 BC), the Battle of Cyzicus (410 BC), the Battle of Arginusae (406 BC), Battle of Tegyra (375 BC), the Battle of Leuctra (371 BC), and the 2nd Battle of Mantinea (362 BC). The Spartans, so far as I have found in reviewing these 51 battles, never made a single creative strategic or tactical innovation. They were sometimes clever; mostly through treachery and trickery.
That said, you have to remember that pretty much all of the primary sources you read on this topic are written by Athenians for Athenians, and less about preserving an accurate historical record for future generations (or even propaganda/promoting internal Athenian solidarity) and more about making specific political points for their own internecine fights. So you should pay more attention to what’s objectively verifiable (things like win-loss records, inputs and outputs) and less about the overall vibe that they present.
Consider using strength as an analogy to intelligence.
People debating the heredity or realism of intelligence sometimes compare intelligence to height. I think, however, “height” is a bad analogy. Height is objective, fixed, easy-to-measure, and basically invariant within the same person after adulthood.*
In contrast intelligence is harder to determine, and results on the same test that’s a proxy for intelligence varies a lot from person to person. It’s also very responsive to stimulants, motivation, and incentives, especially on the lower end.
I think “strength” is a much better analogy, if you want a non-politicized analogy to think about intelligence more clearly. It’s clear that strength has both environmental and genetic factors, and that strength is real.
It’s obviously possible to make a composite measure of strength if we wanted to do so. Further, singular tests of strength (eg dead lifts or something) would correlate well with other tests of strength, so you can cheaper proxies, while also being clearly limited in both normal and edge cases.
However, believing that there’s a single objective and unambiguous measure of strength would be just as silly as believing that some people cannot be stronger than others.
Strength is clearly valuable for getting through life, while not being enough by itself to solve most problems. A good composite measure of strength should have predicative validity. Being stronger is usually better.
Most people don’t build their identities or moral worth upon believing that stronger people are morally superior, though of course some people do.
*(you decline by maybe an inch between 20 and 60, but otherwise it’s completely unchanging over time barring having your legs cut off or something).