Static pressure in fluid dynamics is just pressure, so the parent poster is suggesting a fan that can produce more pressure (in other words, uses more force to move air).
For example, if you're just setting up a simple "fan blows air through filter" setup, more static pressure is needed to force more air through the filter; a fan that can move 30 units of air per minute might not be able to move 30 units of air per minute when an obstruction, like a fine-particulate filter, is placed in front of it.
If you look at the fan used in the article, you can see that the fan blades don't reach all the way to the outside of the cowling, meaning that some air could actually make its way through that gap, and that the blades are large but sparse, meaning there's more room for unwanted airflow; in other words, the air flow you're getting might not all be coming through the filters, but rather flowing around the fan and getting blown back out.
Static pressure is not about fluid dynamics. The name comes from how it is measured. You block the outflow of the fan (for example with a blocked pipe) and measure the pressure difference between inside and outside of the pipe, while the air is static (ie. it does not effectively flow through the fan).
The pressure difference caused by the fan will be different from the same fan but running in free air (not obstructed).
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).
