I don't think that's necessarily true, at least not 100%
If you have low flow and lots of crap going through your live rock, it's not going to magically make that crap go away any more than the bioballs would. It's just going to hide it better.
Not strictly true there..
The difference between live rock and bio-balls when it comes to particle filtration is that when the particles are on the live rock, they are consumed by the many critters which live in the rock, hence the reason why live rock makes one of the best filtration methods possible...That is why you do not see clumps of food building up on the live rock..simply because it is all consumed..what does stay there, and produce nitrate, if any, is also consumed by micro or macro algae on the rock.
Thanks
Niko