Grappling is not very hard to learn. It really isn't. Its organic to the human body. Taking a punch is not a normal organic function for the human body.
But either way, thats not what Nac is saying.
No one ever says that boxers would outgrapple wrestlers.
What we are saying is that, you being a wrestler doesnt mean you are some unstoppable takedown machine.
Avoiding the takedown process is easy to learn, and the better the athlete, the easier the process.
And someone like Tyson, looks like he would be probably one of the hardest men to take down from boxing, just due to his build, stature, speed, and power.