The article is recommending a larger fan to move more air. However, the opposite may be true, as smaller fans tend to have a higher static pressure.
Given that the airflow is only 1/4 of the rated airflow for the fan, it seems that the resistance of the filter is causing a pressure drop across the fan that is fairly close to its static pressure. Adding another fan in parallel to try to move more air likely won't make an iota of difference, as the pressure drop will be about the same. However, giving it a fan with a higher static pressure likely will increase the airflow, even if the rated airflow is lower. Or, add a second fan in series with the existing fan (with appropriate distance between the two to smooth out the turbulence), which could double the static pressure and therefore the airflow. Yes, adding more filters would also help.
There's a reason that most fans in air filters are centrifugal (radial) fans rather than axial fans - they have a higher static pressure.
Since HEPA filter is significant obstruction you want a fan that is designed for static pressure (which is ability to push air against a force/obstruction).
Given that the airflow is only 1/4 of the rated airflow for the fan, it seems that the resistance of the filter is causing a pressure drop across the fan that is fairly close to its static pressure. Adding another fan in parallel to try to move more air likely won't make an iota of difference, as the pressure drop will be about the same. However, giving it a fan with a higher static pressure likely will increase the airflow, even if the rated airflow is lower. Or, add a second fan in series with the existing fan (with appropriate distance between the two to smooth out the turbulence), which could double the static pressure and therefore the airflow. Yes, adding more filters would also help.
There's a reason that most fans in air filters are centrifugal (radial) fans rather than axial fans - they have a higher static pressure.