Because it's like saying, "All my children are boys, so clearly anyone who isn't my child must not be a boy". Or "I like pie, so clearly I must not like any other type of food".
It's true that all my kids are boys (and that I like pie). It's also true that all devices without metal seem to run better than those with metal.
But fact A does NOT prove fact B. Clearly there are kids out there that aren't mine and yet are still boys. And I DO like things besides pie.
All you can truly say is that devices running metal seem to have issues. But it's neither proven, nor "clear" that it's truly metal causing the issues. There are many differences with metal vs. non-metal devices. To say that it's "clearly" any particular thing based solely on the fact that non-metal devices seem to run better is a fallacy. It's a great indication that you're on to something. It's good circumstantial evidence. But that does NOT make it fact. Not until it's proven. Until that point, the best you can correctly say is "...so there's a good chance the problem has to do with metal".
When you say "...so clearly metal has an issue with blurring consistently at high speeds", I think "That dude clearly is clueless". It's a logical fallacy.
Or if you prefer, here's a far funnier, NSFW explanation from a stand-up comic:
