Car de-icers are very different to those used on runways. They probably contain methanol, as well as other glycols. They also probably contain anticorrosion additives designed to protect car engines - a very different scenario to runways. These could be toxic to plants and animals.
However, they may be effective in small doses in a domestic situation, but very carefully controlled. Remember, de-icers are not designed to remove snow - thats what ploughs and brushes are for. Journalists giving LHR staff a hard time don't seem to understand this. De-icers melt surface ice and stop it refreezing.