Notice the severe Peter Panning effect at 3 minutes. The result is that it looks as if the ring is flying above the table, never touching it.
imperfect shadow map reference:
Edit: The paper actually states that "While ISMs may contain incorrect depth values, the resulting errors in the indirect illumination are
small but the computational gains...". Inaccurate depth values leads to the requirements of high depth test delta, which leads to Peter Panning. But I suppose the purpose is to use this for indirect illumination, which is indeed what is stated, in which case it may be ok? There are some cases that have to be avoided, like thin geometry.