It's a case of a little information being dangerous, poor education, and a massive case of Dunning-Kruger.
Fluoride, at much higher levels than found in drinking water, has some nasty effects on the human body. As the saying goes, the dose makes the poison. But people will hear about that, and think that smaller concentrations just mean that the effect is smaller/slower. A lot of people also want to believe in all sorts of insane conspiracies, so this fits in perfectly.
If it wasn't fluoride in the water, it would be (and often is, anyway) some other stupid idea. Like vaccines and autism.
Most people draw a distinction between a phone/tablet vs a desktop/laptop. It's not a distinction that can be easily defined, but most people instinctively categorize them as such.
But regardless, the issue at hand has nothing to do with that definition. The discovery process requires them to process any applicable devices that might have info, potentially including all of the above (and more). His team said they already did his phone, but that the laptop in question is not used by him. His opponents say that he has not complied with the requirements of discovery.
The judge will rule on whether that laptop is included in the discovery order.