I was thinking the same thing. To know that ABC follows DEF by x days with 95% accuracy would be a superpower in investing.

Of course, the problem with something like this is that using it would, of course, ultimately change the market so that x→0.

That's not necessarily true, there are positively reinforcing relationships and negative ones too. So, some strategies, everyone does them and the result becomes less predictable, others, everyone does them and the result becomes more predictable. A common modern example of a positive reinforcement is people pumping meme stocks. There are strategies that the more people know about it the stronger it becomes.