I think the founders negotiated a secondary in series A - took a couple million off the table. I imagine a16z likely had some clause in the paperwork that prevented them from accepting an acquisition offer this early (I'm speculating).
Or rather, I could see how a VC might want to ensure that the offer is a sufficent ROI, either by preapproving a minimum or requiring signoff. But why wouldn't a16z want to take their huge returns? Did their partners see a better ROI anytime soon?
I assumed they went in at an earlier round at a eight or nine figure valuation, so I was thinking it was a 10-100x on their money in a year or so.