The LSBs of a typical photograph are not uniformly random. For example, here's the blue channel LSBs of the input image from the repo: https://i.imgur.com/lOVlcpU.png
If they are uniformly random, then you know something is up.
I should have clarified, I'm not talking about the JPEG artifacts, I'm talking about everything else.
The faint outlines of objects you can see, the varying textures, and the areas of clipping (where the source-brightness was either above 255 or below 0).
There are other statistical correlations not visible in that image - correlations between the different channels, and between the different bits within a channel.
If I showed you the most significant bit of some non-JPEG'd image, you could obviously see that it's non-random (since it'd essentially be a threshold function). If I showed you the second-most significant bit, it would again be non-random, but perhaps less obviously so. As you go through the bits, it starts looking more and more random, but there are still going to be statistical tests you can do to distinguish from true uniform random bits.
JPEG is super common. Having jpeg compression artifacts, even in images that have been converted to a different format, is not something that will raise eyebrows.
If they are uniformly random, then you know something is up.