Level1: HEPA filter still has to be DENSE enough to capture the smallest particles, hence the issue with airflow.
Think about it this way. Imagine our filter is a screen - a traditional window screen made of a mesh of wires. The filtration particle size is dictated by the size of the open space between the wire edges.
If we double the thickness of the wires, but keep the size of the openings the same, then our screen will still filter the same size particles, but will flow worse, as a larger percentage of the surface area is blocked by wire.
Conversely, we could halve the thickness of the wires and maintain the same size of the openings. This would increase the air flow rate through the screen, as less of the screen surface area is blocked by wire. But it still "filters" to the same level as the hole sizes are exactly the same, blocking the same size particles.
Given the right materials and engineering, it's possible to make HEPA filters that flow fairly well. And given the wrong materials (or simple production economics of the way the filters are manufactured), it's quite possible to make a Non-HEPA filter that
doesn't flow well.
And again, just so it isn't misread- I'm only saying that it is possible for a HEPA filter to out-flow a non-HEPA, so that claim isn't immediately BS. But it would take a particularlly good (and likely expensive to manufacture) HEPA filter, and a particularly lousy non-HEPA filter by comparison to make that happen.