if its nitrogen filled and sealed, it means the binoculars is waterproof tight. When no air and water can go into the bino, the lens elements inside the binos will not get fungus. If its not nitrogen filled, there's chance that fungus can grow on the lens elements inside the bino.
However, as mentioned earlier, the lens elements on the surface that is exposed to the outside can still get fungus. the solution is to keep it in drybox when not in use, regardless if its nitrogen filled or not.
as for the bino which u saw, yes.. its can be quite heavy if u intend to use 2hr - 3hrs non-stop for concert.. it's a 12x50, which is quite "large" in size.
I dunno how true are its claims of waterproofness... but still, I would spend a bit more and get a brand which is well-known, at least I know some assurance is there..