There's no rule against being surprised, and especially not against _ever_ being surprised. They are asking people not to express surprise, real or not, when responding to a question, as a courtesy to the person asking. Like: Keep your surprise in check in situations when it's not helping.

I don't think that's too much to ask, just like it's generally considered preferable to not chew with your mouth open when in company, or talk to people with food in your mouth.

They even acknowledge that they expect these rules to not be followed all the time.

I'm surprised that you read it as a general prohibition of being surprised. ;)

Edit: Sorry, wrote my reply before I saw your substantial edits where you clarify your position and distinguish between "positive" and "negative" surprise. I agree there's a better way express surprise. The rule clearly wants people to avoid putting other people down by creating a feeling of "wow, how can you not know this". That's the spirit of the rule, and I guess it could be expressed better. I don't think they'd have a problem if someone managed to turn the surprise into a positive, but to do that, it's good to be mindful that it's often perceived negatively.