I believe the quote "Don't assume malice when ignorance can suffice" works here. Whoever edited probably did not intend on insulting or being rude to you, but was probably editing your post for other reasons and default to their style of writing.
I disagree with your chosen answer, as it displays ignorance backwards. You should enlighten us on what the actual cause was, by talking to the editor.
To directly answer your question, it is not rude to use contractions, but it is less professional from my experiences. I try not to use them, but am not 100% at it.