I think the fact that most of the Gpay issues will have happened after the AWS outage started probably meant that some staff did jump to the wrong conclusion. In fact, if I hadn’t been aware of an issue on Sunday and had a problem on the Monday morning, I’d have jumped to the same conclusion myself.
As to what caused the issue itself, you’ll not get more than they’ve said. It was probably an issue at TfL’s end, but maybe caused by a change implimented by either GPay or Amex, that TfL hadn’t properly understood or wasn’t properly explained to them. As long as no-one is out of pocket then it’s just one of those things that happen occasionally. And I say that as someone who loves to know exactly what happened and gets frustrated when people won’t explain.