It's triple slot. What are you going to put under it to block air flow? There are no multi GPU configurations any more. It looks like the 4090 doesn't have the NVLink connector at all. A sound card or LAN card is not going to block the air flow to a card that large.
Triple fan cooler, sure. And maybe 3.5 slots deep, but there are no fractions(0.25, 0.50, 0.75) to slots when it comes to usable PCIe devices, thus round up to four slot.
One should leave some space for air, as the axial fans on open air coolers have a harder time drawing air in tight spaces compared to the radial ones on blower coolers. That's at least one more slot just for fan breathing room.
5 slots, but more space beneath the cooler is better.
Some people also put capture and sound cards beneath them.
Power supply shrouds affect gpu cooling - the perforated ones do too, though the effect is lessened. The further the fans are from those shrouds, the more access to air the fans have from other sources - like the front and back.
Look at NZXT's H200. Exactly 2 slots of space; dual slot cards will be flush with the psu shroud. It's not going to get any air from the front or the back. But right beneath that:
-psu blocking like half the perforations
-cable management bar(removable) blocking more perforations
All that in the way. It'd be hard to see a high powered, dual slot, open air cooler doing well there - you'd have to put it on a loop or gpu AIO/CLC.
If you're trying to put a halo card in a SFF, it's been years since that's been practical with an air cooler. Water cooling is basically a requirement.
SFF has been one of the best form factors for cooling due to case fans' proximity to components.
People just need to take time planning their component selections, however limited. If some of the current trends(like glass and solid panels) died off, it would have more flexible options.