To me it means that Darkvision does not trump the Darkness spell. "Can't see through the darkness" is a fairly natural way to write that. WotC isn't noted for the clarity of their writing.
Consider the fact that the new wording on heavy obscurement rules in the errata make most obscurement unidirectional. Errata is not for rules changes, it's for ideas the designers had in mind all along but failed to clearly convey. If they've been thinking of obscurement like Fog Cloud as something you can see out of but not into or within, it would be natural to write that "you can't see through Fog Cloud with Darkvision", even though you're clearly intended to see out of it.
And because there exist ways to gain unidirectional heavy obscurement, making Darkness a special case of bidirectional obscurement doesn't really gain you anything from a game balance perspective, since there are other spells that can be exploited just as easily in exactly the same way. And it's more difficult to adjudicate, because you have to calculate angles and line of sight. As a DM I think it's better to just go with the apparent intent and say, "anything within the Darkness radius cannot be seen, modulo truesight/tremorsense/etc."
The difference between Darkness and Invisibility is that Darkness can be used against you (if enemies close to melee range), and Darkness cannot usually be used to be sneaky, since it's a big moving blot of anti-light.
