The Mac mini's $900 base price looks like a bargain, but the source explains that Apple keeps it low by omitting components that laptops must include. There is no display, keyboard, mouse, or battery. A replacement MacBook display alone can cost several hundred dollars, and lithium-ion packs add further expense. The mini also avoids the engineering costs of making a thin, drop-resistant, power-efficient device; its cooling system uses a large circular vent that would be impossible in a laptop.
The source argues that these savings are real but not always passed through to the buyer. Someone without existing peripherals may need to spend more than the difference on a monitor, keyboard, and mouse. For new Mac users who don't already own those items, a MacBook Air can be the better value, since it includes a display and input devices out of the box. The mini makes more sense for people who already have a monitor or want to work from a TV, and it offers more ports and higher-end configuration options than a laptop.
The article also notes that the mini is not user-upgradable, but it can be configured at purchase with more storage and memory, up to a top-end model costing $6,800. That puts it in a different market than the base model, but it shows how the desktop form factor allows Apple to offer both a low entry price and a high ceiling.