I don't actually know how it was done but based on the apparent 'ethos' of makers of impossible bottles, I would guess it's probably a legit baseball and still

I don't actually know how it was done but based on the apparent 'ethos' of makers of impossible bottles, I would guess it's probably a legit baseball and still contains original baseball contents. It seems that these puzzle-makers are creating for the appreciation of their fellow constructors as much as they are for us newbs. I expect the 'good ones' don't rely on easy cheats as much as extreme cleverness, bespoke tools and jigs and painstaking specialized labor. My guess as to the method is the maker partially unstitched the baseball at the seam and then cut the insides into slices. Then restitched the baseball with enough pieces absent to fit through the bottle's neck and leaving a large enough opening in the seam to insert the remaining pieces once in the bottle. Insert the partially open, partially full baseball then insert the pieces, perhaps with double-sided tape pre-applied so they remain in proper position. Then stitch the open seam closed.