There are question about "floating" objects. But how possible make photo with that depth of field in mirror? It is ordinary shot that reflected after, so?
While it isn't certain that that photo was taken in a mirror (I would hazard it is likely it was taken with another camera based on the lighting.) A mirror has no impact on depth of field unless it isn't a standard mirror.
The apparent distance is what is important for depth of field because it impacts the angle the light is coming from. A standard mirror doesn't alter this, so what matters is the total distance the light has traveled. If you are 10 meters from a mirror and take a picture of yourself in it, your effective distance is 20 meters. 10 meters to get from you to the mirror and then another 10 meters to get back.
With regard to the depth of field, shooting though a mirror is not much different from shooting though a window. In the mirror you see a virtual image that is exactly (exceptions follow) the same as if you would mirror the camera position to the other side of the mirror and remove the mirror (and wall etc.).
The key exception in this case would be that the camera and its photographer are part of the mirrored image.
There are more effects added like reflections on or in the glass etc. But none of them has an impact on the depth of field.
But you are right with some respect. The example that is discussed behind your link appears having some unrealistic course of sharpness (do you say so?). They should be lightly sharper when you consider the distance from the lens to the mirror and back a the focal pane.
Unlike a glass ball as in this example. In these cases the focal pane lies within the ball. Unfortunately I don't have an explanation for that effect.