The ASOP states that it takes precedence in case of conflict, but it states that at the top of the ASOP itself, I don't think that there's any equivalent statement in the rules. I think you're correct that the Concealment Loss/Gain Chart is the only one specified in the rules that way.
Speaking of the ASOP, I just noticed one of those "the chart says more than the rules do" the other day. The rules about offboard observers merely discuss how they are set up and how they are used ... but not when they are set up, so one might well presume that, all things being equal, you set them up at the same time as you set up your other forces. (Compare with the rules for Pre-Reg hexes, which do specify that they must be set up before anyone sets up anything.) Nope ... the ASOP actually explicitly tells you to set up Offboard Observers at the same time as you set up Pre-Reg hexes (i.e., before anything else is set up). In other words, it doesn't matter whether you're Attacker or Defender, you won't know exactly where your opponent's units are when you set the Observer up. What is slightly less fair (perhaps) is that the Attacker is not normally allowed to conduct any LOS checks when setting up his Observer so he has to eyeball those potentially critical kill zones .... (Note that the Defender is not guaranteed to get any free LOS checks either, of course, but it's more likely in most situations.)