This is what I thought too, there is nowhere on/in the lug to access this so I really don't think it's meant to act as a screw with a head. It's completely encased.
On the road side of the forum, there was a thread about Bonded Frames. From memory, the radical part was the
order of
assembly in the jigs, and the jig used to push all the tubes together right and keep them there till the glue cures.
My guess is, the AL seat-stay tops are fixed with allen key grub screws that were accessible from the inside of the seat-lug,
all assembled prior to any tube bonding in any lug at all. That join is not completely rigid, and never was supposed to be;
it allows sufficient wiggle room to account for different seat-stay angles for different size frames (otherwise they would have had
to make umpteen more seat-lug variations) and to push the rear triangle into the bottom bracket lug.
What is absolutely critical to keep the stays together and fixed is the brake bridge, so it may be an idea to check that over too.
All that said, I think it is best to have it as wall art, or if you do ride it, close the stays with force and put a zip-tie there.