> It must be noted that customer safety and EMC requirements for the mini PC, a standalone electrical item, differ from those for hardware components (such as the PSU) designed to be inside a PC case. The safety standard suggests that ventilation openings on case side panels need to be less than 5mm in diameter.

...but it's a plastic panel? I don't understand how this helps with EMC.

It's a safety standard, so the requirement is "toddlers can't get their fingers stuck in the fan".