when speaking about expert opinion, we have to remember that dominion is not just pick a set of cards and go; it is also a huge amount of decisions about what order to make buys. SCSN is likely better at picking a set of cards, but it is possible that the bigger issue is the ability to optimally play the game once a set of cards are chosen.
For instance say SCSN takes SP 92% of the time and wins those matches 80% of the time. Does that mean it is the dominant strategy 92% of the time? Not without a lot more priors. For instance, if SCSN wins 75% of his games anyways, there is extremely little information to be gained by his win rate. Further, it may well be that SCSN plays SP better than some alternative cards. Say we could exhaustively have a computer search all possible end game states for each decision SCSN makes after he picks SP and the rest of his strategy. He makes the optimal move 98% of the time. If, with other cards, he makes the optimal move only 95% of the time on average, then cards that intrinsically (e.g. you computed every possible game) are 3.16% better or less will still result in SP being the better call for SCSN, but worse for everyone else.
There are, broadly, two big skills in dominion. Strategic - picking a strategy from the cards in the kingdom and tactical - making correct decisions, particularly with timing, while playing the strategy.
Elite players win too many games simply off raw tactical skill to accurately gauge card strength off their win rate. It is only when they are playing equally skilled opponents that card strength has a larger impact on win rate that player tactical skill. SCSN could almost certainly beat his average opponent most of the time even if he intentionally picked the second best strategy on the board. Flawlessly playing the 2nd best strategy will very often beat marginally flawed play of the best strategy.
This is why Big Money is so strong until you really get to know the game. Your ability to screw up decisions (when should I start greening, should I take two components at $7 or a gold when I will eventually need both, do I reveal this moat against a Margrave or do I draw for 3 good cards, etc.) is relatively low. So even though the strategy, if played perfectly, is weaker than another, also played perfectly; a casual player can come close to perfection (say 80%) with BM strats but only makes 33% correct plays with a complicated engine build. The engine has to be more than twice as strong to come out ahead.
Disambiguation is really hard. I see way, way too much variance in the annual card rankings to say that player skill is sufficient to determine card strength.
That being said, SCSN is totally correct about the best course of action to get better. Improving your tactical play happens when you put yourself into difficult challenges and you get very good at getting the maximum amount out of a strat. Picking individual cards or classes of cards and always playing them will make you much better at playing them optimally.
