If I were one of the people who knew about the success of the trial I would not have thought the stock was priced correctly on the day before the announcement.
The problem is that not all information is known to all market participants at once.
So, we have two correct pricings:
1) What the price is now based on what portions of knowledge each participant has;
2) What the price would be now if everyone had the same information.
I think that's where a lot of confusion originates about "all information priced in"
The people who have the knowledge that, in their minds, justifies a higher price, may be in no financial position to do anything that significantly moves the price higher.