Like often, when an article promises "the best...", that bit is and has to be a lie.
There is no " the best", in general, but one for me and one for you and for another person. At least that made it into the conclusion. As I can't speak about the other systems, I was just checking the X-T2 and was missing two topics I consider worth to mention:
This thing does a bit more than just increase continuous FPS. Reduced blackout, faster AF-C, much longer reach with three instead of one batteries. I really think, it's one of the best grips I had so far, but by a long throw not the most expensive one. He didn't have one for this test (as it appears), but getting a bit better results in speed comes at the cost of a bit more money spent, weight and size. However, I didn't went mirrorless because of size. The gap towards FF DSLR still is huge.
The focus by wire.
Sucks totally. Nice if the birds take care to always fly in open spaces, than even I get a chance to spray and pray. But in any situation the AF shrugs his head, I save a lot of nerves by just putting the camera back into it's bag.
I also disagree with the amount of controls (minus the ones which are redundant or of no big use), but that's just me.
And the 100-400... well, the only lens at the moment for this kind of subject. And the moment already lasts a long year, all they come up with is a super fat and expensive 200/2 which still is not available - I don't know, I would not wax too much poetry about X-T2 at the moment.