Yes. A D90 can display Live View on an external screen, and there are two ways to do it.

1. Direct video output: connect the camera’s HDMI or A/V output to a TV, monitor, or projector with the proper input. In Live View, the camera can show its live feed on that display without tethering software. This is useful for self-portraits, but you’ll still need a remote release or self-timer, and it helps to pre-focus or use manual focus so the camera doesn’t focus on the wrong spot.

2. Tethered Live View on a computer: Nikon Camera Control Pro 2 supports Live View with the D90 on Windows and Mac.

So the article’s advice is basically correct: it can mean using the camera’s video output as a bigger Live View screen, not just standard tethered capture. If your current Linux tethering setup doesn’t show Live View, that’s likely a software limitation rather than a limitation of the camera itself.