As you’ve licensed the item, you’re entitled to leave a rating for the item, and you’re entitled to leave comments on the item… whether they be critical, full of praise, or just some general observations. Buyers have always had that right, and that’s unlikely to change any time soon.
All authors should ideally be friendly and helpful, but if an author is rude on a third party site, which isn’t the official place to get support, then there’s no much that Envato can do. It’s his Facebook group, Facebook provide the option to delete posts and the author has exercised his right to delete your post.
Should an individual or a company delete customer complaints to improve their image? That’s debatable, I’d probably say no, but it’s not always that black and white with stuff from Themeforest and CodeCanyon. You have half a dozen operating systems, using half a dozen browsers, on half a dozen different versions of those browsers, and a theme that maybe uses half a dozen plugins… problems can and do arise, that may not be indicative of a poor quality product or a lack of quality control. The author may feel that your comments were unjustified, inaccurate or that it was unfair to post before seeking support through the right channels. Who knows?!
If you have an issue with the item then you should visit the support page of the item. If you don’t have a problem with the item but would like to contact the author with suggestions for improvement… then you can visit the support page, post a comment on the comments thread or contact the author via a private message through their profile page.