I just inspected the piston top and valves and they show no signs whatsoever of any sort of damage so I do struggle to see how this could be caused by an object inside the cylinder. The damage occurs on two sides of the cylinder wall and if the object moved to both sides, it should also have left signs elsewhere. I've seen engines after objects were dropped inside and it looks awful inside...

After thinking on it and looking at it more, it seems to me more likely to be from piston slap or something along those lines - the location of the damage is consistent with this, being directly perpendicular to the crankshaft and occurring 180 degrees from each other within the cylinder - right where the piston skirts are.
And it's hard to tell for sure but it looks like there are deposits above the surface, not just gouging below the surface.

My current leading theory is something like a leaking injector washed the oil off the alusil coating, allowing the rings to damage the cylinder bore or the friction of the rings dragging caused slap.