How can a diamagnet be stable on top of a single dipole? Earnshaw's criterion being invalid just means that there is at least one static arrangement of magnetic dipoles that lead to stability. However, if you have a point-like diamagnet resting on top of a single dipole it can't possibly be stable because there is no point at which it will have zero net force and stable higher-order derivatives. You need something like a bowl-shaped magnetic field arrangement for it to stay in a single point, or have the diamagnet itself be shaped something like a bowl over the field.
Yeah you bring up a good point... I don't think it can.
But you CAN do it with concentric rings of magnets. Such magnets seem common for this exact demonstration actually. It doesn't look like one of those in the video though.
