HEPA filters may not be necessary - it seems that homemade air purifiers with standard filters can outperform them in clean air delivery rate, arguably the most important factor for a purifier. The air may not be as clean with each pass through it, but it’ll move much more air.
As mentioned in the article these can be quieter than a retail unit. The large fans move a lot of air and the large filter area means it’s easier to pull that air through; both combined mean a lower fan speed is needed for a given throughput.