The most likely explanation is that the listed EXIF was wrong. The photographer later clarified that it was a single exposure and the shutter time should have been about 40 seconds, not 140 seconds. He also used an external speedlite to light the tree with several bursts from different angles, then adjusted color temperature in post.

So the sharp-looking stars are not evidence that 140 seconds works untracked at 16mm. At around 40 seconds on a 16mm lens, stars can still look acceptable depending on viewing size, print size, and where in the frame you look, though slight trailing may start to appear, especially near the edges.

The 600-rule is only a rough guideline, not a hard law. Sensor resolution, enlargement, and composition all affect how much trailing is noticeable. For stricter results, many photographers use more conservative rules than 600. For much longer exposures without trails, you generally need a star tracker.