I don't think the responder necessarily intended to be rude. He was saying that if you engage in the activities that an Agent engages in, then you are acting as an agent and it is wrong to pretend otherwise. You will need to review the legislation in your state but in most states, if you show a property (other than one which you own) to someone who may purchase it and you hope to receive any sort of remuneration, commission, kick back, referral fee, bonus, etc. then you are acting as an Agent. If you are not a licensed Agent contracted thru a Broker, then you are likely in violation of the legislation.
The "duck" expression simply means: If a person or thing performs in most respects like a "duck" then for all intents and purposes, it IS a "duck".