If doing it two days earlier can be considered reactionary...

That doesn't provide causation; you don't wake up on Tuesday, see a price change from Google on Monday, and change the keynote you are giving on Wednesday to rush through a price drop. If anything, it makes more sense to think that Google got word of the things Amazon was announcing at their conference a few weeks ago, and after some careful thought decided to jump Amazon's announcement so that they wouldn't look expensive.

His comment doesn't imply that causation happened in either direction.

I am not certain how to read the comment other than "it isn't possible for it to be reactionary as it happened first", which I believe I corrected (but maybe, as I demonstrated a stronger conjecture in addition, that doesn't count?).

One would assume now that they are priced nearly identically.

Any economist would.

Could Google be leveraging their grasp of the world's free information to predict their competitors' pricing changes?

According to the re:Invent keynote Amazon has dropped aws prices 20+ times over it's 6.5 year lifespan. Google doesn't need to tap into it's records to know Amazon is playing the high volume\low margin game.

Edit: typo

This could be a signal they want to send to Amazon, too "every time you cut your prices, we will, too, and that will just be bad for both of us".