This place has been around so long that GG and I have run a marathon worth of laps around some of these issues!
One quick response to your post, I think one of the important points they make is that the majority of the funds are already "invested" in US assets, so it's not like there has been some major shift from outside the US into corporate coffers. Their point is that it has had a measurable impact on the value of share buybacks, but no other noticeable impact, specifically on real investments.