I don't think you can make a reasonable argument about this when your starting point is theorising based on the Standard Array lol!
None of that makes even basic sense. No one magically knows of a baby will grow up to have 10 STR (I mean man what?!) and further nothing in the lore suggests they value only strength. On the contrary the stuff about finding their place in the tribe suggests there are other ways.
And yes "very dark" is a wild exaggeration. By that standard Klingons are pitch black in tone.
Is there some other metric I should be basing it upon?
IF I take the commoner statblock everything is 10's, which still gives the average Goliath a strength of 12. To be lower than a 10 they need to have a starting score lower than an 8. However, under the character creation rules of both point buy and standard array, Telling me that a score of 7 or less is probably incredibly rare.
Sure, it isn't exactly a scientific, statistical analysis, but it is close enough to give us an idea. If the Goliaths expel the "weakest" members of their tribe who cannot keep up, then they are likely expelling members with a strength of about 10, which is average human. Except, Goliaths all have proficiency in athletics and Powerful Build which treats them as large for carrying. That tells me that a weak Goliath is likely able to carry twice the equipment and supplies that a human would carry, and be able to climb and move through the mountains similiarly to a hobbyist mountain climber or survivalist.
And sure, nothing says they value only strength, but considering they value "athletic competitions" and abandon the weak, then the two biggest would be Strength and Constitution. The mental stats are not highly valued, because (whether this was 5e, 3.X or what) I remember reading that Goliath tribes usually suffer from a lack of leadership, because their fear of aging leads to their elders generally committing suicide by monster. So, physicality trumps everything else as presented in the RAW culture. Sure, you can have a wise leader, as long as they are also strong. And you can have a great craftsperson, as long as they are also strong.
As for Klingons, I have never actually watched much Star Trek, but considering the time period the TV show was made in, the "warrior culture" things I do know, and the visual designs I'm aware of. Yeah, it probably is dark as pitch if examined properly.