What he's suggesting is a search for specific kinds of evidence, that go beyond that correlation. When we are properly skeptical, we have to look at obvious evidence and ask what other evidence would be less prone to confirmation bias. We also have to look for conflicting evidence - purpose look to see what opposes our current position. If we do that with kata, it becomes reasonably clear that kata is not inherently negative (because we have good examples like kyokushin where it seems they produce similar results whether they use kata or not), but it's likely that focusing too much on it has a negative effect.You asked for evidence. The fact that MMA is composed almost entirely of martial arts where kata isn't emphasized or even practiced IS the evidence you seek. The fact that we have yet to see any MMA fighter emerge from a traditional MA base loaded with forms practice further supports that evidence. The only one that comes close would be Machida, but even he wasn't breaking out Uchi-Ukes when he fought.
I have thoughts as to why that focus has the apparent negative effect, and I think a lot of it is non-causal correlation: people who like kata and are drawn to programs that teach using it are probably less interested in competition and strong-contact sparring. That's my theory, and it would be self-compounding, but it's hard to get any real evidence to support or counter that theory, so I'd classify it as a reasonable explanation at best.