Let me explain as I am a mechanical engineer with years of experience in matters such as this.

The BOP is secured to the sea floor with large "bolts". These bolts, and it's weight, are what hold it in place. The BOP is not designed to hold back enormous amounts of pressure. In a typical operation oil simply passes through the BOP and up a pipe and into a ship. In many cases the oil is being sucked up the pipe and there is no pressure on the BOP at all.

In this case, the BOP is being pushed upward - OFF OF THE SEA FLOOR - by the tremendous amount of pressure. When they attempted the top kill they temporarily sealed up the top, meaning the pressure was not being released through the leaks but was instead being held back by the BOP. The bolts that hold the BOP in place began to break loose from the sea floor so they stopped the operation.

But now the BOP is being moved all over the place by the pressure and it is breaking loose. So they either don't back the ROV up far enough to show it or they use the arms on the ROV to hold onto the riser pipe. This way the ROV moves with the pipe and since there is nothing to gauge movement by down there it doesn't look like it's moving at all.

We are screwed and the OP is correct.