Most interesting failure. My guess is there is a plug that inserts into the tube. With other materials, a joint like that would be epoxy bonded, but I assume yours was welded.

It should be reasonably stable, but I fear that the geometry of putting the entire load on the seatstays would tend to pull the plug out, and risk a catastrophic failure on the road.

https://www.ticycles.com in Portland does frame repairs. Perhaps other titanium frame builders too. It is worth contacting them before you scrap the frame. Or, sell it with full disclosure of requiring repairs.

Is that a 700c/650c(24") frame?