Correct, isSupportChat is used to enabled/disabled the Chat feature. Perhaps your account level settings are over ridding your user Chat setting. Can you confirm your account level setting are correct? Also, please take a look at this Developer Forum on how to disable in-meeting chat via the Zoom Web Portal , and via the API
Update account settings
https://developers.zoom.us/docs/api/rest/reference/zoomapi/methods/#operation/accountSettingsUpdate