FWIW, the GP poster is Matt Dillon. He's a well known FreeBSD/Linux kernel hacker and the founder/maintainer of DragonFly BSD and his list of Nerd Cred is legit and long. I'm sure he's forgotten more about network protocols than I ever knew in spite of my kernel patches and Samba contributions. I'd wager he's painfully aware of the ins-and-outs of NAT and IPv6 at a low level.
Don't get me wrong; that doesn't mean he might not be wrong in his evaluation of the protocol. He'd just be wrong on a much more detailed level than I could comment on with any comfort.