It is not a marketing gimmick. Watching 4K videos on a full HD display can enhance the viewing experience. A 4K video has enough information to display four times as much as a 1080p video, so even when downscaled, it can provide a sharper and clearer picture than a native 1080p video. Additionally, 4K videos often have a higher bitrate, which can reduce compression artifacts and improve overall video quality.
However, it's worth noting that the benefit of watching 4K videos on a 1080p display depends on the quality of the video itself. If the 4K video is heavily compressed or has a low bitrate, it may not look significantly better than a well-encoded 1080p video. Additionally, the benefit of 4K resolution may be less noticeable on smaller screens or for content that doesn't have a lot of fine detail.
In summary, watching 4K videos on a full HD display can enhance the viewing experience, but the benefit may vary depending on the quality of the video and the display being used.