Does Slardar counter tanks... or do tanks counter Slardar?

This is something I think about every time I see a Slardar pick. People often say that Slardar is a counter to tanks because you're able to kill hard-to-kill heroes more easily. However, the minus armor effect is the same for every hero, so couldn't you argue that it isn't really a counter? You could pick a tanky hero because you want to be more resistant to that minus armor.

Edit: By tanks, I mean high HP heroes.