I got a theory for why the rock from a lit tank could have more filtration power than the one from the sump.
I think if you had done both in the dark but still using rock from the display and rock from the sump. Your display rock would still have more nitrifying bacteria than the one from the sump.
Reason is the heterotrophic bacteria is responsible for decomposing dead organisms in our tanks, during the process nh4 is produced and nitrifying bacteria will be in bigger numbers in close proximity to were Heterotrophic bacteria is at work. I can observe this behaviour in the current experience I’m doing where Heterotrophic bacteria breakdown phytoplankton in to basic elements inside a reactor, one of the observations I’ve seen was production of nh4 from the vessel where all happens. I wouldn’t be surprised the reason my experience was successful at removing photosynthetic dinoflagellates was due to a explosion of herbivore bacteria.
edited: in conclusion I think your rock already came super charged from your display and the light may not be doing a difference regarding nitrifying bacteria. This could explain why
@Coxey81 rock took so long to process nh4, probably most of the nitrifying bacteria was in the display