The annual GROWTH is 20%, that's including hunting etc. Not birth rate. So with everything staying the same, the population will double every 5 years.
According to the study a sow drops 1.6 litters per year for around 10 baby pigs. Assuming 2 million of the 4-5 million are female of breeding age... and really nature favors females especially in herd animals so more than 50% of the population will be breeding and dropping babies.
To keep a population in check you would have to eliminate more than half of the females, and before they drop babies. Otherwise the existing females are dropping more potential breeding females than hunting etc removes from the breeding pool.
The fun part, even as the population grows and hunter success improves due to more targets, the % of population removed by hunting decreases. There's only so many hunters, and so many pigs those hunters can take. Unless you have a matching number of hunters as breeding females who take a matching number of pigs as will be born every year, the population will grow.