First of all Sid is not running Thumbnail ads on LinkedIn, your ad is displayed as an 1:1 ad. The reason why the ad looks like this is due to the fact that the ad has got organic engagement, and while you are looking at it without being part of the target audience it appears as an organic post instead of how the actual ad would appear.

Below is an example of how the LinkedIn thumbnails appears as, this is not how your ad looks like for the people that are part of your target audience.