id think amex would have talked to marriott.. maybe hilton offered more, or marriott was less enthusiastic.. or, perhaps we can be optimistic that marriott was ok with amex doing hilton too, and it is simply part of ongoing discussions. amex can always try and beat the deal marriott has with chase.
finally linked my accounts, current confirmation email from SPG says >
we don't anticipate launching a newly combined program until late 2018 at the earliest
which again adds to official/executive/etc quotes i posted earlier