I worked on torsion balances, gravity measurements, and gravity gradiometers for my PhD. Getting just a factor-of-two increase in sensitivity is significant--getting 4-5 orders of magnitude better would be incredible and also next to impossible, given the other factors to account for.

The author considers environmental gravitational noise, but you have to also consider that any movement of the instrument in a non-uniform gravitational field will create a signal. If you consider Earth as a perfect sphere, the gravitational signal of a B-2 that the author considers is equivalent to the change in gravity from a vertical displacement of 0.2nm. This is about the size of a water molecule. Vertical stability at that level is unachievable.