The legislation says you can't ask for a breath sample unless you are lawfully stopped, so if the stop is not lawful, then the demand will also be not lawful.
Again, this is best practice in terms of mandatory alcohol screening. Best practice in numerous dozens of countries around the world works most effectively if every driver passing the checkpoint is waved in and every driver stopped at a checkpoint is tested. Similarly, in terms of mobile mandatory alcohol screening, every time a driver is stopped, they are asked for a breath sample. If the stop is illegal, there is no right to make the demand, under the proposed legislation.