Crunchbase shows AngelList's funds raised at $400M - there were 3 follow-on rounds with undisclosed amounts after their $24M Series B. (I have to revise my estimate of AngelList's valuation up and my estimate of the percent ownership of PH founders/employees down after this - with $400M raised, they're almost certainly worth at least a billion, and Hoover's equity share would be worth < 1%, also in line for a talent acquisition.)
I certainly can't speak for a16z in this particular case, but in other talent acquisitions the VCs usually prefer cash to shares in the acquirer. If they wanted shares in the acquirer, they would've invested directly in their funding rounds. Among other problems with taking shares, it opens them up to potential conflicts of interest (particularly relevant for a16z + AngelList), and it leads to having large positions in companies of which they don't have board seats or any significant visibility.
Ryan Hoover gets "FU" paper money in the form of AngelList shares; he has to wait for AngelList to have its own liquidity event before he gets to realize any of those gains. Still, if his own startup is foundering, that's probably a better bet than going down with the ship.