If you are at least attempting to put your points forward for a meaningful debate or conversation, it is your responsibility to present them well so as to reduce the chance they are misunderstood. But, if you want to be like every other slob on the internet and abdicate on your responsibility of your half of the conversation, I can stop debating with you as well.
As a person who spends a great deal of time debating controversial subjects... interpretation is a tricky business.
There are many, many people who will purposefully misinterpret a statement to prove a point, and then will claim that it only occurred because the original "misinterpreted" message was worded poorly or something to that effect. Often times they're actually not worded all that poorly.
Now, I do not think that you purposefully misunderstood what he was saying, but I do think that your bias took away your objectivity on the subject, which made it much easier to take offense to a questionably-worded message. Agenda was a poor word for him to use to accurately describe his meaning. His next line however used the word motive, which is a good deal more accurate. You have to take all the segments of his message into account to assure that you're not cherry-picking out a specific piece to make the post look worse(inadvertently or otherwise).