But how can you quantify that? There is no way to prove it, the market cannot say "I wasn't moved by this, I was actually moved by that and this part was actually just negligible."

Isn't it all subjective in the end because nobody really makes their trades with verifiable notes expressing the exact reason. So we can only guess right?

> how can you quantify that?

Precedent and timing. Rates-related news is always going to massively shift the market, and the market shifting right after the jobs report is a pretty clear signal.

Moreover, S&P holding course wasn't new information–there was zero evidence of anyone pre-trading a rebalancing, which means the market didn't expect S&P to materially change its rules.