Vergeben getting Simon right shows that his first initial leak was likely legitimate. Sure he could've guessed it right, but I think saying he had accurate information prior to E3 is a lot more likely. But Vergeben getting Simon and the rest correct has no weight over Isabelle or the Gen VII Pokemon, since they seem to have come from different sources. Now, Isabelle may have come from the same source since Vergeben feels so strongly about her, but the fact that she was mentioned separately sometime after the initial leaks leave her 'source' up in the air.
Vergeben's 'sources' have claimed that the Gen VII Pokemon isn't Decidueye or Lycanroc, but even Vergeben himself seems to feel shaky on those claims. Helps show that the source behind the Gen VII Pokemon newcomer is different, and thus may not be accurate. Vergeben has gotten some right, but has also gotten plenty wrong in the past. Mimikyu getting deconfirmed doesn't hold much weight for me, either, since I never saw Mimikyu as a likely playable character. As far as I'm concerned, the Gen VII Pokemon is still between Decidueye, Lycanroc, and Incineroar. The only three choices I've ever really seen as viable.