If that were the case then anti's mk (in which I play anti more than any other person in this community) would wreck me. He is the best mk vs diddy, or was possibly even possibly the best player vs the character at one point when he was super active. He knows every in and out of my playstyle and has beaten ninjalink a few times as well.
Well, you should take it up with your fellow Diddy mainers, the majority seems convinced that a day will come when MK will dominate them harder than he does now.
And about playstyles, hmm... I play against RATED's snake all the time, I mean ALL the time. I know him in and out, what Snake's attacks do to me, their hitstun, the ranges, damage output, when will they be stale enough to not kill me, where I should DI, what to do with grenades, landing patterns... Seriously, there was even one time I thought Kirby vs Snake was dead even. But what happened? Well, I was too accustomed to his playstyle and used THAT as my analysis for the MU. When I played against another Snake pro from PR (DooM, yeah what a common name), I lost because he was mixing it up really well, in ways that I didn't realize RATED should've been doing to me all along.
That doesn't mean that Kirby vs Snake is still even, it just means that playing against RATED all this time gave me the illusion that the MU was even, but in actuality, it was a 40:60 all along. RATED now destroys whatever Kirby decides to show his face in his brackets, and I destroy every Snake that I find in mine (obviously, here in PR and not in USA). We both know both MUs extremely freakin' well, but it still doesn't mean it's even.
Now, I'm not saying that Diddy vs MK isn't even, you and all the diddies/MKs should know that better than me. But, just so you know, illusions tend to happen during friendlies, especially when you play a lot against them.