My understanding is that HF that short the US Treasury will need to buy the spot in order to deliver at the maturity date. Won't that increase the demand for UST and reduce yield instead?
I wouldn't give these explanations much credence. You can always concoct some explanation ex post. For ex. if we see increased shorting and a spike in yield you could say "the futures traders are the smart money, they predicted a spike in yield and sure enough it happened". Or "it's the Global Macro funds again, shorting futures". Sounds intelligent but who knows if it is true? Best to be skeptical of these narratives IMO.