I can't help but think that they already were in a sort of informal alliance.

It took Google dropping prices for both AWS and Azure to drop... clearly they were both skimming profits off the top for the last while.

Of course, a company does need to profit and I'm not accusing either of them of doing anything wrong, I'm just trying to say that these sorts of alliances form accidentally and naturally; both companies hit a competitive price and then hardware gets cheaper; software gets better... they could drop prices, but why cut into profits when there's no need? Inertia is a powerful force. It takes something actually happening to kick those price drops into action, generally.