No, UVB helps produce more trichomes because the plant attempts to use trichomes as a sun skin so to speak.
MH normally = more potency while HPS normally = higher yields
The MH has more UVB than HPS which causes more crystals. So some people like to use MH in all or parts of their flowering cycle as a way to get the best of both worlds and receive potency + high yields.
However if it is true and the lens from a reflector blocks the UVB, there's almost no point to use MH in flower unless you aren't using a lens.
The reason the Metal Halides are helping is their Blue wavelengths.
Blue is good for potency.
UV is close to blue on the spectrum as well.
So, the reason it may work to increase is either it is damaging and the plant protects itself. Either way the adding of the MH helps in the the last days. If you have both a MH and a HPS I would put both on if that makes sense with your situation.