CustServe told me "That is True." in response to my interpretation. They then used an example in the PHB of where this held. "That is True" is the answer to my question, meaning my interpretation was correct in its entirety - he didn't even feel the need to elaborate.
Either that, or CustServ was confused by your question, since you worded it in a pretty leading and confusing fashion.
You asked...
"Page 55 of the PHB strongly suggests that a spell which does damage of a given type also gains the keyword of that type. To whit: "For instance, a power that deals acid damage is an acid effect and thus has the acid keyword." Literally, if it deals acid damage then it gains the acid keyword. Is this a correct reading of this passage, and is the implication general (if a power deals x damage type, it gains the x keyword)?"
While I can tell that the relevant part of the question is the "is the implication general" part at the end, I can very easily see someone from CustServ reading your question and thinking,
"Well, duh, flip to the PHB and Acid Arrow has the Acid keyword. Why is he even asking this?"
Without asking the more specific question (such as starting with your (A) and following it up with a "(B) Based on the above, do the Illusion spells from Class Acts gain the Psychic keyword?"), I see your response as less valid than the direct answer of "No, it does not" to that same question.
I don't think CustServ is gospel by any means, and I think it was just sloppiness in the Class Acts article that is leading to this confusion. (Oh boy, there's a shocker - the new version of Dragon Magazine might have some inconsistencies! So might CustServ!) That said, I do disagree with your decision that your answer is valid and his isn't, because you gave them a wall of text and tried to lead them into saying what you wanted to hear, and he didn't.