This take me some time to research how to make tinymce in readonly mode.
The key point here is set readonly to 1 or 0, not true or false. For example here is my init code:
tinymce.init({
            selector: "#html-textarea",
            menubar: false,
            statusbar: false,
            resize: "both",
            plugins: [
                "textcolor image link preview code table media noneditable"
            ],
            readonly: status, // **status hold value 0 or 1, NOT true or false**
            toolbar: 'preview | undo redo | removeformat | cut copy paste | formatselect fontselect fontsizeselect | forecolor backcolor | bold italic underline strikethrough subscript superscript | alignleft aligncenter alignright alignjustify | link unlink image media | code table | bullist numlist | outdent indent blockquote'
        });