You may be misunderstanding Marbi's intention of what he said. He wasn't necessarily blaming serebii. He was just saying it would have been nice to have serebii reporting and keeping up with the VGCs is all.
The thing is, when the forum is new, there will be ALOT to discuss from the beginning, but the things to discuss pretty much becomes repetitive and hard to maintain with new things, which will explain the inactivity on other forums. The thing is, I can almost guarantee (just don't use this against me later lol) that during the World Championships and other ICs, the forum would be active.
And, VGCs has actually become an ongoing thing, so the earlier the forum is up, the better. It wouldn't really make much of a difference when it comes up~
On another note, can't wait till the forum goes up!