Would have to disagree with some of the posters above. I dont know the dynamic of the relationship between the two but I were to guess, it would be a strong one (Southampton, Dutch connection plus made him Dutch captain). So, Koeman probably knows he can make such a comment about Virgil, he is strong enough to be unfazed, its more like a message to the other members of the Dutch squad that they need to keep standard highs.
And from a liverpool perspective, anything that helps Van Dijk to break away from the cohort of the best defenders in the world to just the standout best defender in the world is welcome. So, if this helps him to focus just that little bit extra, so be it.