Yes, concealed/HIP units are resolved as Area Fire. Its not explicitly stated until 163.3's preamble...
One of the most difficult aspects of the artillery system for many to justify hertofore has been the halving of indirect fire vs a concealed/hidden unit. How does concealment protect the target from indirect fire when the firer never sees the target anyway? The obvious answer is that concealment does not make a target harder to hit so much as it makes it less likely that a spotter would have any reason to call fire down on an area to begin with. The halving effect was merely an attempt to compensate for the omnipresent eyes of the game player who can see concealment counters and thus predict the presence of a target that a real observer would have little reason to suspect.