Same here.
There are a bunch of investors out there that I know, like, and trust.
Also slightly less importantly, there are investors I don't necessarily trust.
The composition of the syndicate is an important signal.
Big difference, angel and VC are not interchangeable in this discussion and the article explicitly mentions VCs, not angels.
He does say that, that angels cooperate more with each other than VCs do. So yes.
But I think the salient point is whether the investor is asking who the other investors are so they can cooperate with them, compete with them, or collude with them. And that depends on the investor, not whether they are an angel or a VC.
There is a lot more than gut judgement involved in deciding to give somebody I just recently met a large sum of my money. I don't let other VCs dilute my judgement but I certainly listen to them when they have more knowledge of the relevant facts about a company, its people, its product, or its market.
Note that I'm an angel investor, so I try to make sure the cost of due diligence is proportional to the amount I'm investing. If I were a fund and writing million dollar checks, I would spend the time to do all the due diligence myself.