It’s probably in the api lock in. Amazon can switch to another provider once they’ve got a better deal elsewhere, and customers need not notice. They’re commoditising twilio, which is not a great place to be in, for stock.
(Alternatively: Amz ends up buying twilio and the market was wrong :) )
I don't think this is what's happening but would be an interesting acquisition strategy. Spook the market into offloading Twilio, driving the price down before they make their move.
They can't really easily move elsewhere for 2 way texts. It'll require porting numbers worldwide in every country away from Twilio without any downtime
(Alternatively: Amz ends up buying twilio and the market was wrong :) )