This plugin DOES NOT add anything to your post content. You can look through the code of the entire plugin, and you will not find a single mention of the word "Tweet".
That is being caused by another plugin that you may have active. (Most likely some sort of Social Buttons plugin). I checked your site and basically this plugin is adding the buttons in your post content. The scripts are being called in the footer. Unlike other plugins, twitter's tweet button actually has a valid link tag. Which is visible in your feed.
Try disabling your social plugin, and you will see that your feed issue will be fixed.
As for why the og:description was showing Tweet... that is because we pull from your content (the first few words). If another plugin is hooking into your content, then that data will be pulled.
Hope this makes sense.