There is a second video from that same account on that very issue, which I watched but did not link. My recollection is that many such holes, in other sites as well, feature these cores, and spiral grooves. The one that has been very well studied is in a British museum, which makes it accessible, and it is also a high quality artifact, making study of it revelatory.

I do not specifically recall the cores being described as tapering, and have to confess that I simply assumed that cores from a tapering hole would be tapered. The images shown of such cores do not seem to have been carved away by a tool that did not bend to fit a taper, however, and seemed to be well fitted to the holes, which would lend credence to their being tapered.

Good point!

Thanks!