It looks like the $1B figure is a pre-money valuation for the $500M raised.[0] If so, that results in a 2x exit for those investors.[1] This assumes those investors don't have rights that would entitle them to more than their pro rata of the acquisition proceeds, such as special liquidation preference rights that would entitle them to more than a 1x liquidation preference. It's these kinds of special liquidation preference rights that have attracted some attention for helping "juice" valuations, particularly with unicorns.[2]
[1] Math: $500M invested at $1B results in those investors owning 1/3 of the company. Assuming pro rata distribution of the acquisition proceeds, they would get $1B in a $3B acquisition.
