When I was researching this I knew of the description of ${parameter:+word} in man bash, but when I read what it said, I "knew" that it couldn't be the explanation for this _expression_ as it made no sense. So I kept on looking for some exception case that "looked" like ${parameter:+word} but couldn't find one and hence my list post.
I experimented with every conceivable version of ${@, ${*, ${1, etc ending in address@hidden, "$@"}, $*}, $"*"}, etc and none of them made any sense.
That it only makes sense in response to an old known bug - makes sense.