Good point, although I think you mean concentrating your gravity dilutes your IBU's. The flip side is that a more dilute wort will lead to higher IBU's, which may not be desirable in certain styles. For my really hoppy brews, my calculated IBU's are usually in the 60's-70's, so even if I lose 5-10 IBU's I'm still at the upper limit of the bitterness threshold. Plus, I get to use the topoff water to rinse the hop trub in my kettle for some extra hoppy goodness.More dilute wort during the boil will lead to better hop utilization and higher IBU's. But they were talking about adding water to wort post boil to correct gravity. This would certain lead to lower IBU's because of 1) higher gravity during boil and 2) post boil dilution of alpha-acids.