That isn't right. It notes the U.S. government doesn't collect comprehensive statistics, then it does a decent job citing many alternative data sources that all point the same way: A Brookings Institution estimate, trends over time from National stats of Portugal, Ireland, France, and some scattered datapoints from Spain, Netherlands, UK, Czech Republic, renunciation of citizenship numbers.

Even if it was purely % stats, you don't need a baseline figure for the claim in the headline (Americans are leaving the US in record numbers) just that the number is going up. There's plenty of solid info that this is a real phenomenon. What's uncertain is the magnitude and significance of it.