Looks like a set of very unfortunate circumstances. I would have used the quick install guide as well. Doubt I would have even looked at the fine print. REI is not a fault since they are under no obligation, other than being nice, to inform you the incompatibility, since it is stated in the fine print. Thule makes super racks, I have had a few, with no issues; but if this is a well known issue, they should sticker the boxes NOT FOR USE ON SUBARU unless the receiver is upgraded. Feel for you.

Edit: One thing you post doesn’t make exactly clear is where the rack or hitch failed. Did the pin break releasing the rack or did the rack hitch metal break, or the Subaru hitch detach slamming the rack down?