Smith didn't put up huge numbers until his junior year (though he did put up better than .5 points per game in his first two years which seem to have been cut short for whatever reasons...injury?) while Miller is putting up close to a point per game as a freshman.
It's 100% entirely possible that he will not be able to carry that over to the NHL, if he even continues it through the rest of his freshman season, but again...a freshman putting up these point totals is intriguing because it's likely he will continue developing (at least, we hope) but is still excelling at a level that is new to him. I'm more excited about a freshman doing so well than a junior doing so well, if you get my point. It makes me think it's more likely he has the talent to develop into a point producer at a higher level as well.
That said, even if he doesn't become a big time offensive player at the NHL level, I'd be ecstatic for a 40 point 1st pairing defenseman who is a great athlete, plays in all situations, and just owns the shit out of his minutes. Basically what we all kept expecting out of McDonagh (and what he gave us for several years)