Was it pressure treated lumber? Because it sure wasn't the joinery that kept it from cupping.
No, it was plain old framing material with one coat of redwood stain and no sealer.
But have you ever noticed that a solid wood top on a table (like a dining room table) is never glued down. And in fact the screws that attach it are in elongated holes? Or in clips in grooves? That's to account for wood movement.
Yes, but he is not building anything as big as a dining room table. It's only 16" wide. I have never seen a coffee table, or a curio table, or a desk, or anything that size with a floating top.