It's a great piece. You found it. That tells the whole story. The weathering and water makes it look smooth and clean. It' the real deal. I don't think those holes were drilled with a modern bit. A common modern drill bit will not drill a hole in a rock. It takes a carbide masonry bit, which come in 1/16"-1/8" graduated sizes.
That’s a bucket list find for a lot of people I would be ecstatic to have found that.
Slate is very soft material. Natives drilled it for thousands of years without carbide bits.