Oh, wow... I followed the link to the other article where they describe how they came up with such a figure:
> “For instance, the T. rex population was likely about 20,000 at any one time, but the 95% confidence range — the range of numbers in which there's a 95% chance the true number falls — is 1,300 to 328,000. In other words, when the T. rex total is calculated (which includes population density, population size at any one time, generation time and total number of generations), the number of T. rex individuals that ever lived could be anywhere from 140 million to 42 billion, the researchers said.”
If you click through that sentence to the linked article [1], you'll discover that it's a pretty misleading one. The 2.5 billion estimate is the total estimated number of adult Tyrannosaurs that lived during a 2.5 million year period (68-65.5 million years ago).
The estimated maximum population of living adult Tyrannosaurs at any one time, on the other hand, is ~20,000, with a 95% confidence interval from 1,300 to 328,000, according to that article.
> “For instance, the T. rex population was likely about 20,000 at any one time, but the 95% confidence range — the range of numbers in which there's a 95% chance the true number falls — is 1,300 to 328,000. In other words, when the T. rex total is calculated (which includes population density, population size at any one time, generation time and total number of generations), the number of T. rex individuals that ever lived could be anywhere from 140 million to 42 billion, the researchers said.”
> — https://www.livescience.com/number-of-tyrannosaurus-rex-on-e...