Was Erik Gudbransson not a sure-fire top pair D? Bogosian? Adam Larsson? Even the guys I've listed, who were in the running for #1 overall, often fall short of the top-pairing mark so I'd need much harder evidence than the words of Samuel Tirpak, whoever he is, to believe Nemec is basically generational talent. Regarding your last sentence, that is fair enough.
Regarding Cernak, you said "Nemec is a year and a half from being drafted, and is already playing in Slovakia's best mens league.". So Cernak is a natural comparison in that sense. It's not something we haven't seen before.