The difference is that blue whales are social animals, living in groups (whereas T-Rexes were probably solo hunters), and also that the living zone of blue whal

The difference is that blue whales are social animals, living in groups (whereas T-Rexes were probably solo hunters), and also that the living zone of blue whales are a large fraction of Earth oceans (whereas the article you mentionned applies to North America dry land, a much smaller area than the cold-to-temperate oceans). So, in the case of blue whales it is understandable that the low ten of thousands is not the healthy number of individuals that they used to be (hundreds of thousands).