I don't think free will is an illusion, it is only a part of the human psyche. But it is ideally free, in other words, if we were completely freed from all kinds of intrusions and influences upon our will. As an ideal, free will is not therefore to be considered illusory. It is real to the extent that you make it real in your life as an effective force toward personal progress.
If you want the MBTI to limit you, you are free to do so. This knowledge can cut both ways. Or you may see the MBTI as only doling out labels for what are only temporary states of being. With knowledge of typology you are free to pursue different, possibly better, ways of being.
"I absorb energy like a sponge everywhere I go. It allows me to see the world and my purpose in it." Zak Bagans, Ghost Adventures (INFJ)