Therefore, a statement like 'this method lets me play any concept I want' can only be objectively true if that method allows any combination of 3-18 for six scores to be generated by that method.
Point-buy does not allow that. Rolling does.
Interesting point. D&D was based on a concept that each stat was based on a independent bell curve for each stat. As Gygax was a insurance underwriter he loved his numbers and he developed a system that he must have felt approximated nature.
But the problem with his method is that it does not actually simulate nature when you take it down to the individual level. Look at Olympic level athletes. Here is where you find the 18s of the human race. But look what you don't see. You don't see a human that is the best at everything. You don't even find many humans that are best in more than one thing. A weight lifter is not as fast as a fencer. The fencer can't beat the marathon runners, the Marathon runners can't even beat sprinters.
Because in real life individual bodies at better at one thing than others. You build fast twitch muscles, then you lack endurance. You have twice the muscle mass than the average, you loose flexibility.
So independent die rolls don't reflect reality. But a point buy system can. Mainly because you can approximate getting pretty good at something, or being well rounded but you can't make a superman.