I would be suspicious of any claims to returning to "historic levels" - hunting whales got serious in the 1800s, lighting and powering the industrial revolution, long before we counted the numbers sensibly.
The article addresses this. > Prior to the late 19th century, blue whales were simply too big and powerful to pursue... Roughly 380,000 blue whales had been killed in all, and the species was at 0.2 percent of its initial numbers. Given that they couldn't be hunted until fairly recently, and having a decent idea of how many got killed, it should be possible to get very reasonable estimates of pre-hunt populations.
Indeed. Prior to steam powered ships and harpoon guns, only the so called 'right whale' (https://en.wikipedia.org/wiki/Right_whale) was hunted by Western nations (by rowing out to them in small boats, sometimes from shore). The right whale is slow, and most importantly, most of them float after being killed.