I don't think it proxies that well. As the article points out, a16z raised a $1.5 billion fund, some top-tier firms might come close, but most of the mid- and bottom-tier VCs operate with the fund sizes in double-digit millions.
Also, for a highly contested B round, A round participants or brand-name VCs will get preference over a generic nondescript firm with a Sand Hill Road address.
Somebody from YC might correct me on this, but I think a16z's participation in automatic round upon admittance to YC comes with drag-along rights, which they can choose to exercise at either A or B rounds. So they don't have as much of a problem of missing on quality dealflow that others will miss on if they choose to skip A rounds.