I don't think this is true at all. The CBP certainly has a record of your entries into the US, but not your departures. If I exit the US by land to Canada, no record exists for CBP. I'm fairly certain, too, that scanning your passport when leaving the US by air provides the airline with evidence that you have a passport and ability to enter the country of destination, but no data is transmitted to CBP.
It was my understanding that the US government received outbound flight manifests that included citizenship information. I imagine that this is primarily of interest to US CBP (for determining whether non-nationals overstayed visas, etc.).