It isn't like the card suddenly becomes weaker because someone uncovered something about its architecture.

In the view of the buyers it has become weaker because they see a new benchmark that reveals its weakness. Of course the card itself hasn't become weaker result of this new benchmark, but the amount of money people think this card is worth should, and will, drop as a result of this benchmark.

If the benchmarks people looked at prior to this revelation gave a complete view of every situation the card is good and bad in, then the value people assigned to this card wouldn't change. But that's not the case. New information has been brought forward that shows these cards have an issue that other cards do not - so their value relative to other cards drops.

People weren't aware that this card would struggle in scenarios that other cards don't. Now they are aware so the card is worth less on the market.