Is there sin in saying, "The Prophet, may Allah bless him and grant him peace, forbade poetry, and poets are without intention [or purpose]?"

If you repent from speaking about religious matters without knowledge, then there is nothing held against you, and Allah is Forgiving and Merciful. It has not been established that the Messenger absolutely forbade poetry; rather, he prohibited excessive engagement with it lest it distract from what is more important. It is narrated in Sahih (definition) Muslim that having one’s stomach filled with pus is better than having it filled with poetry. Imam An-Nawawi clarified that this dispraise applies when poetry overtakes a person and distracts them from the Quran and the Shariah sciences. However, if the Quran and Hadith (definition) are predominant, then memorizing a small amount of poetry does no harm. While some scholars have deemed poetry absolutely reprehensible, the correct view is that it is speech: its good is good, and its evil is evil. The Prophet listened to it, commanded it, and had the Companions recite it to him. He only condemned what was blameworthy, such as obscenity.