Correct, Exclusive members can use the Exclusive Sales section - that's not indexed.
As accomatic pointed out, there is a way to avoid being spidered, by posting the domain as suggested.
http://www.dnforum.com/rules.php#10
"There's a solution for those avoiding spiders, you may use example(.)com or example(dot)com."
If you post it that way, and someone replies on the thread with the exact domain in their post, feel free to report it and a moderator or myself will correct the post to reflect the way you preferred it be posted.
That may be what happened to your thread that apparently was indexed.