That particular example aside, useful information for many HNers: Earn outs in tech are routinely earned out. They're negotiated with the assistance of very smart, very highly paid specialists, too.
A fairly common outcome (anecdata from friends) is that the earnout is virtually in the bag at the 50% point and after that they are mildly frustrated with thumb-twiddling while waiting for the clock.
From my anecdata, the acquiring company is using the earn-out as more of a test that what you are saying at acquisition time is true. It augments due-diligence.
Meaning, if you are right about the business, these milestones are trivial to hit. If you don't accept the earn-out, it's a signal that something you are saying is either wrong or being misinterpreted.
A fairly common outcome (anecdata from friends) is that the earnout is virtually in the bag at the 50% point and after that they are mildly frustrated with thumb-twiddling while waiting for the clock.