The old time solution for this problem is to use p-clamps and attach a "standard" rack to only the seat stays.
However, they do not inspire a lot of confidence for heavier loads. I think what the manufacturer is objecting to in your example is clamping on a carbon seatpost. If the bike has an aluminum seatpost, I don't see a lot of objections to the rack you posted as long as the loads stay light. No manufacturer is going to say seatpost clamp racks are okay due to the liability issues involved. A poorly designed or installed seatpost clamp rack can be a real problem and the manufacturer has no control over that.
FWIW, there are a lot of solutions these days for bike packing for gravel bikes that don't require a rack and standard panniers. Big seat packs, frame packs and bar bags can hold a lot and distribute the weight and width better for a bike that might see some single track. These bags also have an advantage in rougher terrain, in that their mounting hardware is relatively soft and stretchy nylon. Bumps won't directly transmit the force into spots on the frame tubing that aren't designed to take those kinds of point loads.
If you want some examples of the kinds of bags available, look into Relevate Designs.