This boot.img has been modified to unsecure meaning you can adb remount, not stock boot.img for any Coby I have seen.
This boot.img is rooted, meaning all you have to do is adb remount, from there you could adb push su and chmod it, no need for the root method you posted as your boot.img is unsecure.
Others can't get the method you posted to work, cause their boot.img is stock ie secure, so adb remount doesn't work for them.