You don't have to submit IELTS but it's strongly recommended unless you are a native English speaker. If you choose to send other proof of your English proficiency, you are allowed to do that. If they ask you for IELTS anyway, you can refuse to give it and they will then estimate your points based on your documents. However, that can mean that you end up with 0 points for English.
One person I heard about who didn't have to do IELTS had been working as an English teacher for some years. Another person who was asked for IELTS after giving documents had a university degree in English so really, you never know.