i'ma bet that frame was designed NOT to mount a rear rack, but to be solely used for bike-a-packin'. those braze-ons on the bendy seatstays are probably intended as fender strut or seatbag support mounts.
decorated with an array of mounts for hauling your daily provisions and overnight gear
sounds like a light load is intended, so no need for super massive racks.
at most you might could mount one of those carbon-fiber flyweight rear pack racks.
Seatstay-mounted bottle bosses, plus three-pack mounts at the top and bottom of the down tube
no mention of rack mounts, not something they'd forget to mention.
according to the video on the website, the designer dude says it's designed for "all-day dirt rides" and "carrying the bare necessities for quick and dirty bike packing escapes."
NOTE: at 1:39 in the video the dudester states:
we added bottle bosses to the seat stays on both sides of the frame
see bottle cage mounted to seat stay at 1:50
(just me, or does it look like hipster don't spend much time outside of his cubicle??)
https://surlybikes.com/bikes/ghost_grappler