The key is that the whole scene was not captured in a single 180-second exposure. According to the photographer’s description, the bridge was photographed separately in 2–3 shots at different exposures, then blended later with the star-trail exposure in Photoshop.

So the 180-second exposure was used for the sky/star trails, while the brighter foreground and city lights were exposed independently to keep them from blowing out. This is a common technique for high-contrast night scenes: expose the sky for the stars, expose the foreground for the artificial lights and detail, then combine the images in post-processing.

In short, the “trick” is multiple exposures plus blending, not one long exposure that somehow holds both perfectly.