This can indeed occur naturally, as people who wade creeks do find similar stones with the culprit still inside the depression doing its work. It looks like a creek stone that had a pebble/cobble in a depression, and over time, with water running steadily over it making the pebble/cobble wobble around in the depression, gradually abrading the surface of the depression as well as the surface of the pebble/cobble. As a result, the depression was at first enlarged as the pebble/cobble became smaller. At some point the little cobble got small and light enough that the hole it was grinding in the depression also became smaller in diameter as it got deeper. The little stone eventually bore all the way through. That isn’t to say a person who needed a mortar or more likely a butting stone wouldn’t find such a stone useful too, and picked it up to take it to their shelter to use in food prep. It just looks like a naturally holed rock more than it does the mortars I have seen. Mortars usually have regular shaped depressions, but this has a very irregular shaped depression. Nutting stones can be more irregular since the depression is just used to hold the nut, not to grind them.
For example, there is a place not too far away from me called Peewits Nest. You can walk through that narrow gorge and the water in most places is only about a foot deep. After a rain the water looks like youre walking in chocolate milk. Then youll see a kid jump from 20+ feet up right down toward that hard flat stone bottom. Sploosh! They disappear and pop back up.
There are wells ground into the solid stone floor by a similar action. The well is only a couple of feet wide and how they aim at it through chocolate milk Im not sure but its deep enough to catch them safely.
Many years ago I used to do something similar up in the UP along Lake Superior. The water there was quite clear. We could jump off the cliffs there quite safely because the wells were huge, deep, and easy to see.