yes, you need write a statement to explain that.

For example, you can state you have studied and lived in US for a few years, your daily study are in English, you discuss project with your professor in English, write report and publish paper in English, etc.
Also, you listen and speak English in your daily life, too, you speak English while you go for shopping, banking, etc. you listen English radio, watch English movie/TV, etc.
In conclusion, you state that you think your English is as good as what level you declared.
Hope this gives you the idea.
Actually, they just need such a document, otherwise, your application documents will be considered incomplete. However, if you declare your English at high score, it is better to make the statement looks nice.
By the way, you may still get the notice to ask you do a ISLE test. You may just ignore and not to take the test, they will determine your score by their impression based on your supporting document, and you still have big chance to meet the language requirement.
However, there could be some risk if you let them to rate your language score, taking the test or not depends on how serious you really want the application to be successful and how much effort you willing to spend for the test.