Yes, it's not really backing them any more than the equally non-committal statement from the Norwegian National Security Authority is. The Reuters headline is not borne out by its article.
"We have no reason to doubt" is not a weasel statement. "At this stage" is a weasel statement, but only in the sense that it means "We haven't investigated this or seen any evidence ourselves, and until we do we'll go with the line taken by Apple and AWS."
This can be taken as meaning "The US has given us details and asked us to deny them" - which makes no sense, IMO.
Or "The US has given us no details" - which is rather more likely.
Nobody said it was a weasel statement. Nobody said it was their job to back these companies. The claim is that, counter to the title, they have not backed these companies.
This is a pretty mild statement, would not say it backs Apple and Amazon.